How to PERMANENTLY Get Rid of Foggy Water in a Fish Tank

Small, seemingly insignificant changes in the water chemistry can run out of control in a sealed environment such as an aquarium.

Sometimes the result of this turns out to be cloudy water in the fish tank which signals an imbalance that can potentially culminate into your fish dying in the worst of cases.

Obviously, this can be the bane of any fishkeeper’s existence.

The causes of this nuisance are generally the fault of a bacterial bloom that has cropped up for whatever reason in the tank.

This may happen in what seems like an instant, with the water becoming foggy overnight.

It’s also fairly common in situations where it’ll crop up and become suddenly cloudy after 1 or 2 days of setting up a brand new fish tank.

This is often seen in a goldfish or a Betta fish setup, though it’s not the species that are at fault.

Still, If your water keeps getting murky and misty or ends up milky white after a water change there are solutions.

But first, we need to examine the cause behind the issue in both saltwater reef or freshwater setups, and even in a simple fish bowl, for that matter.

What is causing aquarium cloudiness and what are the underlying reasons behind foggy water?

How to deal with it and how can you prevent this from happening in the future?

We’re going to Aquanswer those questions.

But first here’s a snapshot of a foggy fish tank that’s not even severly affected:

By veekerz

Remember that when you’re facing this, you’re not alone.

It’s a really common issue and it doesn’t necessarily reflect poorly on your ability to care for your fish.

Here’s why the water in your freshwater or saltwater tank has gotten cloudy:

  • Blooming heterotrophic bacterial colonies
  • This is the most common reason behind milky-looking water.

    Bacteria blooms are a normal part of the nitrogen cycle that every aquarium goes through.

    Sometimes they also happen after a cleaning session.

    At other times you can experience this right after a water change or while there are still no fish in the tank.

    The haziness of the water is caused by bacterial colonies establishing themselves.

    The problem occurs when there’s an imbalance of nutrients that results in aggressively multiplying microorganisms.

  • Microbubbles
  • There’s a lot going on in an aquarium and with all the working parts of the equipment there can be some side effects.

    One of these is the microbubbles.

    These are incredibly small pockets of air that flood the aquarium, causing a cloudy appearance even if the chemistry is overall fine.

    They can be created by certain gear mechanisms, usually Hang-on-Back power filters in freshwater aquariums or protein skimmers in coral reef tanks.

    Note that this doesn’t mean the equipment is not working properly.

    The dangers of microbubbles depends on the severity and setup.

    They need to be addressed as soon as possible if they’re severe enough to cause clouding because large amounts of microbubbles can clog the gills of aquarium fish.

  • Hindered or slowed down nitrogen cycle
  • A messed up nitrogen cycle is often caused by a sudden abundance of organic nutrients in the aquarium.

    A certain type of bacteria that decompose the organics immediately start reproducing to compensate for the overabundance of food.

    In a short period of time, the bacterial bloom becomes so severe that the microorganisms overpopulate the water column, causing it to appear hazy white.

  • A chemical reaction causing precipitate as a byproduct
  • This concerns coral reef aquariums the most.

    Because of how complex the chemistry in a saltwater fish tank is, sometimes we forget to do the extra calculation while changing the usual portion of water.

    This could result in an unwanted chemical reaction that produces a byproduct that looks like microscopic white particles that “snow” on everything in the reef tank.

    More often than not this byproduct is calcium carbonate.

  • Unwashed Substrate
  • This would really only crop up immediately after filling the tank the first time.

    There’s a lot of contaminants, too many to list, that can end up in an unwashed substrate.

    If your setup is brand new (literally minutes old) this is a likely cause.

    This case is rather obvious to diagnose, even for a beginner in the hobby, and I won’t be discussing it much more throughout this article.

Take note that if there is a green tint to your aquarium and it looks somewhat cloudy, it will not be addressed by any of the above. This green coloration is generally caused by an microalgae bloom of some kind. Visit the link to learn if it’s harmful to aquarium inhabitants and how to deal with it in no time.

How to Quickly Eliminate Foggy Water in a Fish Tank?

Though not always, sometimes a cloudy fish tank, that seems to have developed suddenly, signals that there’s an imbalance in its internal nitrogen cycle.

What exactly should you do in this situation?

The solution will change dependent on the trigger, but there are standard tools and techniques available once the problem is identified.

To get rid of foggy water in a fish tank you can:

  1. Set up a UV aquarium sterilizer
  2. Clean 35% of the substrate every 3 days
  3. Reduce the feeding to once every 2 days
  4. Replace no more than 10% of the water with remineralized Reverse Osmosis Water
  5. Change your aquarium salt mix
  6. Sprinkle Epsom salt before adding a salt mix
  7. Add polyester filter floss
  8. Use a different aquarium substrate
  9. Wait it out

1. Set up a UV aquarium sterilizer

Here on this pic you can see the dramatic improvement in the water clarity in 7 days, using UV sterilizer:

By Ryan S

I only recommend getting a UV aquarium sterilizer to treat a case of cloudy water that has been going on for weeks or months in the fish tank.

If the water in the aquarium keeps getting foggy for this long, then, perhaps, an outside intervention to forcefully restore the bacterial balance is needed.

Another situation in which the purchase of one as a solution would be adequate is when your freshwater tropical aquarium, or reef tank for that matter, is so severely affected that you can barely see the fish and decoration.

I’m talking about a dense milky appearance.

If this is not you, you can skip to the next section.

The way the UltraViolet sterilizer works is by circulating the bacteria-infested aquarium water through a narrow tube with a UV lamp inside.

The lamp irradiates UV light, bombarding the bacteria’s DNA with high-energy particles, causing them to mutate.

Said mutations will prevent the bacteria from multiplying further, quickly restoring the balance in the water column and clearing things out.

Ultraviolet aquarium sterilizers come at a greater price than the other solutions listed here, however, they are the most efficient method for removing unwanted bacteria in the water column.

A UV light unit will get rid of the foggy water in a matter of days (and sometimes hours), where other methods will take up to weeks or even months to start showing results in the fish tank.

For most hobbyists, the overall cost of a single UV unit is worth the long-term benefits it provides for an aquarium.

By getting one you’ll have immediate control over future bacteria blossoms, outbreaks of free-floating algae spores (causing the aquarium water to become green), and also over questionable tap water before putting it in the fish tank.

Another snapshot showing the effects of use of UV filters:

By Ryan S

A UV sterilizer can also be periodically run in an aquarium as a successful preventive measure against different nasty parasites, such as ich.

These sterilizers are also commonly used by many hobbyists in their quarantine tanks, meant for monitoring new fish that have recently come from the store.

To get the most of your unit you need to know the Wattage of the lamp that would best suit your aquarium size.

You should also make sure it comes with its own water pump that has a low GPH rating (gallons per hour), as the bacteria need to be exposed to the UV lamp for longer periods of time while passing through the tube.

The best aquarium UV sterilizers are designed with both in mind.

Visit the link to check which product has the right lamp and water pump for your aquarium size.

2. Clean 35% of the substrate every 3 days

This course of action only applies as a solution to established fish tanks that have long gone through their full nitrogen cycle but are still struggling with cloudy water.

Why would you clean 1/3 of the substrate but not the whole thing though?

To answer this question I first need to point out why you should be cleaning the substrate.

The misty appearance of your fish tank’s water implies, that there’s an abundance of nutrients going on in there.

When these are not fully employed, the leftovers signal to a certain type of bacteria it’s time to multiply and consume them.

Said nutrients are usually coming from degrading fish poop or leftover uneaten food that got stuck in the substrate.

So to remove the bloom and clear up the water you need to remove the food source these bacteria are consuming in order to multiply.

As we mentioned above, however, the beneficial bacteria (the ones you actually want to have in abundance in your aquarium) also live primarily on surfaces.

If you clean the whole substrate you risk to remove the good nitrifying bacteria along with the excess nutrients of the bad consumer bacteria.

This could potentially hinder the nitrogen cycle even further and you’ll end up back at point zero with, again, a foggy aquarium.

This is why I recommend that you clean 35% of the substrate and wait for 3 days to give the good bacteria a chance to restore their lost colonies.

After that clean another 35%, and keep doing that until you’ve had the whole substrate cleaned.

I know it sounds slow and tedious, but it’s a safe and efficient way to restore the aquarium’s bacterial balance and clearing up the water eventually.

Note that after you’re done it could still take another week or two for the ecosystem to fully stabilize.

3. Reduce the feeding to once every 2 days

This technique also applies to fish tank owners who have had their aquarium going on for a while yet it still deals with murky white-ish water.

It’s often that inexperienced fish keepers feed their fish with more food than needed.

I used to be the same, and I justified it with fear for my little pets because I didn’t want them to go hungry, ever.

Long story short freshwater and saltwater aquarium fish are relatively hardy because in the wild you can’t expect to get fed daily with a juicy amount of bloodworm or algae wafers out of the blue.

For this reason, it’s actually better to feed less than your betta or goldfish can eat.

Otherwise, the leftovers will sink to the bottom where they will start to degrade. This in turn potentially leads to ammonia spikes that are pretty deadly to aquarium inhabitants.

A bacterial bloom caused by overfeeding can arise 1 or even 2 days after the leftovers were left to sit in the aquarium’s substrate.

A white misty fogginess in the fish tank’s water signals just that – too much organic matter is not being put to good use.

This method is best combined with the above one.

Note that your fish will be absolutely fine if they don’t eat every single day.

4. Replace no more than 10% of the water with remineralized Reverse Osmosis Water

This section applies to people who are trying to treat a freshwater aquarium that continuously gets misty water.

Part of the problem here is that water changes larger than 10% can disrupt the water column and give advantages to bacteria.

That being said, from now on you should probably use RO / DI (Reverse Osmosis / Deionized) water for all of your water changes if your fish tank keeps getting murky and hazy.

That’s because I’m guessing you are using tap water for water changes, and more often than not it contains high levels of organic matter.

Excess organic matter will induce the formation of milky white water in the aquarium.

When you perform a water change with tap water the non-beneficial bacteria get to work immediately and they start consuming nutrients directly from the water column.

This, in turn, causes aquarium cloudiness, because the colonies can quadruple their numbers in minutes and become visible to the naked eye.

The advantage of RO water is that it doesn’t have the unnecessary excess nutrients that a freshwater aquarium would not benefit from.

When water goes through RO / DI filtration it gets stripped from basically everything and is mostly left with H2O molecules.

This, however, removes the good minerals in it as well, and most aquarium inhabitants get their minerals through the water (unlike us humans).

You need to remineralize the water to make it fish-tank-friendly again, by using additional minerals.

The company Seachem has two products for remineralization that I have been really happy with.

One is called Equilibrium and the other is Replenish.

You can easily find both products in your local fish store or better yet online.

Replenish is targeted towards community tanks, where Equilibrium works better for remineralizing aquarium water for planted tanks.

Usually, fish stores also sell RO / DI water for about 50 cents per gallon.

This is fine, however, let’s assume for a moment that you have a 55-gallon aquarium and roughly change 20% of the tank’s water each week.

In this example, you can do the math and you’ll see that in roughly 3 months the RO/DI water filter will pay for itself.

In the long run, you will actually benefit from getting your own RO / DI filter (if you can call 3 months of aquarium keeping “long-run”).

I do have some experience with RO/DI filters and one of the cheaper ones I ended up buying was this budget-friendly unit (link to see it on Amazon). I’m fairly pleased with it.

I also have a complete guide on the best ones that I’ve ever worked with, and though it’s suited towards reef tanks, the listed RO/DI systems will work just as well for a freshwater aquarium.

5. Change your aquarium salt mix

This method is targeted towards reef aquarium users so if you are an owner of a freshwater aquarium, go over to section number 7.

Anyway, sometimes we want to diversify and try out new options when remineralizing our reef tank’s water.

There are two types of aquarium salt mixes that can fog up the water in a saltwater fish tank.

The first one is probiotic salts, which contain live bacteria.

These, when used in small water changes of under 10%, are okay and won’t cause clouding.

However, when used for a larger water change, there’s a significant chance of causing a bacterial blossom.

The other type of aquarium salt mix that can make your reef tank’s water milky is the one that simply needs more time to dissipate.

These will likely have it mentioned on the instructions label that it’s normal for the water to become hazy during mixing.

6. Sprinkle Epsom salt before adding a salt mix

This is again for people with saltwater reef aquariums.

Did your aquarium’s water become a milky cloud right after you added the salt mix to the RO?

And the white coloration actually comes from particles that look like tiny snowflakes?

That’s most likely a formation of calcium carbonate precipitation.

I did some tedious research and it turns out that this effect occurs because of some rather complex chemistry that I can’t really explain in Layman’s terms.

To simplify this – whenever you’re changing the water of a saltwater tank you want your RO/DI water to have enough magnesium before adding your salt mix.

If the salt mix settles or dissolves before the magnesium you end up with a huge build-up of calcium carbonate.

To prevent this you should just add some form of magnesium to the RO water before you add the salt mix.

A very cheap solution here would be Epsom salt, which is essentially Magnesium Sulfate.

It has been my experience that it dissolves kind of slow and you need to actively stir the thing for at least 3 minutes.

When you see that it’s fully dissolved add your salt mix and carry on with your water change.

This very simple trick effectively prevents the calcium carbonate precipitate and your saltwater aquarium won’t experience a nasty 4-day long cloudiness.

7. Add polyester filter floss

Filter floss on its own is an excellent tool to expedite the process of clearing up a foggy fish tank on top of whatever else you are using.

It clears up any excess detritus very well as long as it’s changed regularly.

In fact, polyester fiberfill is one of the best methods of mechanical filtration for an aquarium system overall.

In the case of microbubbles, a good filter floss may solve the problem entirely.

The mechanical filtration provided by the microfibers is enough of a wall to also filter out bacteria if that’s causing the water’s fogginess.

This is definitely a good addition to any tank and should be a mainstay to ensure water clarity even after the cloudiness has disappeared.

Note that this will not solve the underlying problems behind the fish tank’s cloudiness, but it will act as a quick and efficient way to give you crystal clear water clarity.

8. Use a different aquarium substrate

Certain types of substrates can end up dissolved within your aquarium, creating a murky or misty environment.

Aquasoils used in freshwater planted tanks, for example, deplete with time, because they’re designed to provide the live aquarium plants with vital nutrients for maximizing growth potential.

However, these substrates are often messy and can turn your fish tank into a “dirted” version of itself.

Some hobbyists do this on purpose in order to simulate the natural biotope of the fish they keep.

However, most people that are new to fishkeeping go with an inert substrate that has a beautiful appearance to it.

The inert ones often do the work for aesthetics, but some of them, if not thoroughly washed beforehand, can contribute to a cloudy aquarium with their fine particles.

This is normal to a certain extent and usually clears up on its own after settling.

However, if a substrate that’s too light is chosen this can take a bit too long and can potentially compromise the short-term and long-term health of your tank.

In my personal experience using black diamond blasting sand works best as a decorative substrate.

It settles easily and is heavy enough to not get stirred by enthusiastic fish movement.

On the other hand, it’s not that heavy to compact and crush a plant’s sensitive roots.

If you’d like to keep live aquatic plants in a nutrient-rich aquasoil, however, you should know what to pick from and how to avoid messing up the color of the water.

I’ve compiled a good guide on the best ones if you want to check that out.

9. Wait it out

The nitrogen cycle in new fish tanks has a break-in period during which bacterial imbalances can cause cloudiness issues.

If not aided by external factors (such as bottled beneficial bacteria starters) said period could last for weeks.

This is a natural part of the overall establishment of an internal ecosystem in an aquarium.

If your aquarium is new and it has just been set up, then your most reliable course of action is to wait for the fish tank’s system to completely establish its nitrogen cycle.

In other words, foggy water, in this case, is a natural occurrence that may or may not happen and getting rid of it requires no action on your part.

Monitor the water parameters once every 1 or 2 days and also the water itself to see if it begins to clear.

For example, a solid proof of a freshwater aquarium that’s still cycling are readings that show:

  • More than 0 ppm of ammonia
  • More than 0 ppm of nitrites
  • 0 ppm of nitrates

Even if the water test reads only one of the aforementioned it’s to be considered as a signal for the ongoing establishment of bacterial colonies.

This is one of the major reasons why it’s generally not recommended to change out aquarium water during cycling.

If the cloudy water occurs in about 1 or 2 days after a new fish tank is set then you should carry on and simply wait it out.

If it’s happening in an already established tank, then something else may be slowing down the completion of the nitrogen cycle.

What Exactly Caused the Water Cloudiness in the First Place?

The initial causes of foggy water in a fish tank vary, however, the mechanism that is triggered is almost always the same.

Perhaps, in 9 out of 10 cases, bacterial blossoms are to blame.

Here’s what is causing aquarium water to become cloudy:

  • An excess of organic nutrients causing a heterotrophic bacterial bloom
  • Aquarium equipment responsible for microbubbles
  • An imbalance of reactive elements causing a chemical reaction
  • Microparticles and dust

That being said, let me explain each of these processes in detail.

1. Excessive amount of nutrients

A healthy aquarium system will have all kinds of bacterial colonies to support it.

The two main types are heterotrophic and autotrophic bacteria.

The former prefer consuming strictly organic matter to survive, while the latter can synthesize their own food from inorganic compounds.

In an aquarium heterotrophs will decompose organics, releasing ammonia (NH3) in the process.

The ammonia will be then picked up by the autotrophs and get converted to nitrites and nitrates in a process called nitrification.

Autotrophs are the beneficial biofilter in an aquarium and they are the ones protecting live fish from ammonia or nitrite poisoning.

That being said, the ones responsible for fogging up the aquarium’s water are the heterotrophs.

They can reproduce at a relatively fast pace of 15 to 60 minutes while the autotrophs take between 16 and 24 hours to double in numbers.

This means that heterotrophic bacteria are realistically able to completely overpopulate the aquarium’s water column overnight.

Following these thoughts whenever there’s an excess of organics in the aquarium the heterotrophs will aggressively multiply to decompose them.

This happens so fast that they are unable to attach themselves on surfaces and instead saturate the water column, causing cloudiness with their sheer numbers.

The decomposing process is called “remineralization”.

Carbon dioxide (CO2), and inorganic nutrients such as ammonia, phosphate, and nitrates are released through the remineralization of organic matter.

Here are the common sources of organic nutrients in a home aquarium:

  • Each time you feed your fish, the food that doesn’t get eaten sinks down to the bottom of the tank where it starts slowly degrading.
  • Each time a fish produces waste, it remains inside of the ecosystem, where it starts decomposing.
  • Dead and rotting flesh in both plants and fish
  • Another source of organic matter is the tap water used to perform water changes in a freshwater aquarium. This is the reason why you can see a brand new aquarium’s water clouding up and becoming visibly milky, even if there are seemingly no other organics present.

2. Filtering gear with in-motion parts

Occasionally, aquarium equipment for filtration such as HOB filters or protein skimmers can produce millions of microscopic air bubbles and therefore cause cloudiness in a fish tank.

In tropical aquariums with freshwater fish more often than not it’s the power filter that produces the microbubbles.

If you have any doubts that this may be your case, it’s worth turning off your HOB filter and waiting for an hour or so.

If it was the culprit you’ll notice a significant clarity in that time.

Often the reason behind this effect is in the way the unit is built.

This does not mean that it will not perform its filtering duties correctly.

It’s worth noting that rarely this could also happen with a canister filter if the tubing is not properly connected.

How about a saltwater tank then?

In a reef aquarium the usual suspect behind a cloud of microbubbles would be the protein skimmer.

A protein skimmer does that as part of its job – it produces microbubbles.

However, sometimes the build of the unit may not be as flawless. This could cause a leak of microbubbles right back in the tank, which could make the water look very misty.

3. Calcium carbonate as a result of insufficient magnesium

Every time a reefkeeper mixes beneficial salts with RO water they should make sure that there’s enough magnesium in the mix beforehand.

Not only the magnesium should be put in the water first but it should also be completely dissolved before the rest of the salt mix is added.

Otherwise, calcium carbonate could precipitate, forming small particles that will eventually cover the aquarium’s internal surfaces.

These look more like dandruff or snow if anything, so it’s easy to recognize and diagnose the issue.

The cheapest way I’ve found to add magnesium in the mix is with Epsom salt.

Epsom salt is magnesium sulfate in its nature.

It’s a rather slow dissolver and it needs a bit of stirring.

There’s another magnesium alternative that dissolves in seconds but is also a bit pricier.

That would be Magnesium Chloride, which is not easily findable over the counter, so here’s a link to it on Amazon.

It’s worth noting that sometimes the intrinsically interlinked pH, kH and Calcium content in a typical tropical saltwater aquarium can be in a supersaturated state which again would produce precipitation.

To look behind the equations, I suggest that you read this article.

4. Fine particles and dust

The reason behind getting murky water thanks to fine particles is often a substrate that hasn’t been thoroughly rinsed before being put in the aquarium.

Most substrates will be dirty out of the bag, so it’s always recommended to rinse them.

Another reason for mechanical contamination of the water can be questionable aquarium decor.

Some objects are not made to endure underwater conditions despite the advertising.

It’s always recommended to have a decent mechanical filtration in a fish tank.

Only introduce a new substrate to your fish tank after cleaning it.

Why Are my Fish Dying Right After the Murky-Looking Water has Appeared?

Ideally, cloudy water should abate within 3 to 5 days with little consequence to your fish.

However, this is not always the case and due to certain chemical processes.

Here’s why your aquarium fish have been dying ever since the tank’s water became foggy:

An aquarium going through a period of foggy water, that’s caused by blooming heterotrophic bacteria, can experience fish deaths, because of the chemicals released in the process.

When heterotrophic bacteria blossom they are converting higher than usual amounts of dead organic matter into ammonia, reducing the water’s pH in the process.

The bacterial blooms happen out of a sudden giving the water a cloudy appearance.

The sudden change can cause a spike in ammonia and a quickly increasing water acidity.

Both of these conditions are known to be lethal to aquarium fish and other aquatic animals, who may die right after the imbalances have occurred.

Heterotrophic bacteria also compete for oxygen with fish when in the process of blossoming.

As you can see there is more than one reason for dying fish when the aquarium becomes cloudy:

1. Hindered nitrogen cycle – In an established tank, the nitrogen cycle could be impeded by a previously dead fish that has been decomposing for a longer period of time.

When this is combined with fish randomly dying then you can be certain that the cause of the mortalities is rotting organic matter.

Dying leaves of freshwater plant matter can also cause this effect in a heavily planted tank.

Too much uneaten food stuck in the substrate is another common cause.

If you find that the ammonia levels are dangerously high (anything above 0.25) it could be a clever move to add bottled beneficial bacteria to the tank.

This will kickstart the establishment of the colonies required to nitrify the ammonia into the much less harmful nitrates.

It’s worth mentioning that sometimes the ammonia readings may remain at 0 during the phenomenon of milky-white aquarium water.

This could be because there are enough live aquatic plants in the tank to take advantage of the excess inorganics (the ammonia).

Plants in an aquarium prefer ammonia as their primary source of nitrogen for growth and will take that over nitrates when it is readily available.

If your tank is not planted, but the ammonia readings still remain 0, the bloom was most likely caused by the organics introduced to the system via a water change.

In that case, the release of ammonia is negligible.

2. pH swings – One of the reasons for performing on-going water changes in a fish tank, aside from removing a bunch of nitrates is to also keep the acidity of the water in check.

Bacterial activity in the aquarium will steadily lower the pH with time.

Fish can endure a steady change in water acidity.

However, during a bacterial blossom the pH can violently drop.

It’s worth noting that acidity changes on a logarithmic scale.

In Layman’s terms, this means that 6 pH feels times more acidic to fish than 7 pH.

Sudden pH drops are very dangerous to both freshwater and saltwater aquarium setups.

More fragile aquatic inhabitants may not withstand the pH swing and perish.

3. Oxygen deprivation – During their remineralization process of decomposing organics, the heterotrophic bacteria use up oxygen in the aquarium.

This is usually in negligible amounts in a well-balanced and established fish tank.

However, during a bloom the competition for oxygen becomes vicious.

The denser the milky look of the aquarium’s water is the more severe the problem becomes.

You may witness your pet fish gasping for air at the top of the aquarium’s surface.

This is one of those cases where installing a UV sterilizer would be adequate.

The device will eradicate most of the bacteria in a couple of hours, whereas waiting it out could take up to weeks.

Don’t forget that in the meantime you can increase the water surface agitation to allow for quicker and more effective oxygenation.

Why Are Bacteria Blossoming AFTER I Perform a Water Change then?

It may seem counterintuitive but a water change, even in an already blooming tank, can actually be harmful to the health of your aquarium and a fruitless endeavour.

If you experience this in a freshwater fish tank then you can be certain that a bacterial blossom is behind it.

Here’s why the aquarium water keeps getting cloudy after a water change:

The issues with aquarium water becoming foggy white after performing a water change come down dechlorination.

Every time tap water is being dechlorinated before a water change it becomes bacteria-friendly.

This, now dechlorinated water, also has organic compounds in it, which are the main nutritional source for heterotrophic bacteria in an aquarium.

When you pour the new batch of water in the tank the heterotrophs start feeding directly from the water column.

This new nutrient-rich environment triggers their aggressive reproducing causing the blooming of the bacteria.

They multiply so fast and in such vast numbers that they overpopulate the aquarium’s water column.

The result is a fish tank that keeps getting cloudy every time you change a portion of its water.

Each water facility takes preventive measures against bacteria which is held at bay via chlorine or chloramine in our tap water.

Once those are gone, what is left is basically a pristine and ideal environment for heterotrophs to quickly feast on its organic contents, unabated by competition.

There are even studies suggesting that heterotrophs manage to survive in chlorinated water, which means you may be actually inviting more of them in your aquarium during a water change with tap water.

In an aquarium the heterotrophic bacteria reproduce very fast compared to the nitrifying autotrophs.

Heterotrophs multiply every 15 minutes while autotrophs need more than 16 hours to do that.

This is why the white coloration of the water appears suddenly.

In about 1 hour the heterotrophs have already expanded their colonies up to 16 times.

As dechlorination is vital for a successful aquarium, the only variable that can be changed here is the source used for water changes.

Why is There a Bacterial Bloom Just 1 or 2 days After I’ve Set my New Aquarium?

It can be fairly disconcerting to have the issue of aquarium water clouding up right after setting up a new fish tank.

There is, however, a reason behind cloudy water appearing in new freshwater or saltwater aquariums:

The nitrogen cycle in a new fish tank begins with heterotrophic bacteria remineralizing available organic matter, degrading it to ammonia.

In the beginning of this process there’s no balance established in the aquarium.

The heterotrophs multiply so aggressively that their numbers become visible in the aquarium water, causing it to appear milky or cloudy.

Heterotrophic bacteria multiply to double their numbers in minutes so this phenomenon always appears to happen suddenly and water becomes visibly cloudy overnight.

The full bloom will likely take place 1, 2, or 3 days after the new aquarium has been set.

Both freshwater and saltwater aquariums can experience this even with no fish present in the tank, because there are still organics available in seemingly crystal clear water.

Because the heterotrophic bacteria use oxygen in the process of remineralization the risk of suffocating fish in smaller aquariums or fish bowls is much higher during the initial bacterial bloom.

Speaking of small fish bowls it often appears that Betta and Goldfish owners are common victims to foggy aquarium water. Which brings me to the next section.

Does This Happen More Often in a Betta or a Goldfish Tank?

In fact, it seems so, but that has nothing to do with the fish themselves.

Here’s why it seems that betta and goldfish aquariums get affected more often by water cloudiness:

Bettas and goldfish are more likely to be kept in smaller enclosures, usually due to false advertising in the fish store.

First-time fish keepers can easily be misinformed and often go for either of these species as their first choice of fish.

However, tiny closed environments can easily be overwhelmed by bacterial blooms causing cloudiness and water quality issues for the aquarium’s inhabitants.

Goldfish are also notoriously messy and produce large amounts of waste relative to their body size, which contributes to further worsening the effect.

Among this and other reasons, fish bowl aquariums that are usually used for housing bettas or goldfish are not recommended and should be upgraded into a filtered system as soon as possible.

Small fish tanks of 2 or fewer gallons are the worst offenders here and the water must be changed regularly to avoid this.

Therefore, betta fish and goldfish, are not to blame for the common issue of cloudy water in their tanks.

Here you can see a photo of a 10 gallon goldfish tank, with ashy water that appeared couple of days after the tank’s been set:

By boulderv14

How Long Before it Clears Up and Will it be as Sudden?

It’s definitely troubling to wait around for things to get better in your aquarium, worried that harm may come to the livestock inside.

Here’s how long it takes for a cloudy aquarium to clear up:

Though it appears out of sudden, murky water caused by a bacterial bloom in a tropical freshwater, or saltwater reef tank will usually take up to 7 or 10 days to clear up.

However, it should be noted that in a number of cases the condition may persist for months.

If the tank is still cycling, new fish are being introduced or other similar environmental changes that increase the organic nutrients available in the system happen, you may experience hazy white water for 6-8 weeks.

It will occasionally clear and come back until the bacterial colonies become balanced.

During this period it’s highly recommended to use water test kits to ensure habitability and safe ammonia and nitrite levels.

After a water change in both freshwater and saltwater reef aquariums, it is common for initial cloudiness to occur that should not last for more than 5 to 7 days.

The organic matter in the new water is normally not enough to sustain a heterotrophic bacterial bloom for longer than that.

However, other issues outside of changing the aquarium water may be occurring if the haziness lasts for much longer.

In reef tanks in particular, water clarity may also be impeded by coral spawning.

This is an annual process wherein the coral reproduces for 1-6 days and up to a week after a full moon.

It shouldn’t cause any real issues and is quite a beautiful process once you’re aware of what is going on.

It should be noted that by using a UV sterilizing light that’s the appropriate wattage for the particular aquarium and water volume, it should only take the cloudiness 1 to 4 days to completely clear up, depending on severity.

The habitability of your water is improving from the moment the UV sterilizer is turned on, even if you don’t visually see its effects instantly.

I’d like to stick to that as my recommendation for persistent long-lasting blooms that reoccur or a case of very densely white aquarium water.

It has always been effective for me and my tanks and it’s not at all risky unlike taking the chemical approach.

8 Effective Preventative Measures to Ensure Stability

Now that you’re aware of the causes and how to remove cloudiness in an aquarium, you may be asking yourself if there’s a way to prevent this in the first place?

Though certain natural processes are unavoidable, the actual damaging types of cloudy water are entirely avoidable.

Take a look at the tested ways to prevent a fish tank from becoming foggy:

  • Feed less than what you think the fish can eat
  • From my observation over hundreds of aquariums, overfeeding is, perhaps, the number one reason behind any imbalance of nutrients in these closed ecosystems.

    To keep organic matter in check you should feed less than what you think your fish could eat.

    Be precise and try not to go over a set small portion of food.

    If the meal sinks down to the bottom and your fish did not see the batch – don’t sprinkle another pinch.

    If you do not trust yourself with precise dosing, you can get one of those cheap automatic fish feeders.

    They will feed a pre-set portion of food on a set schedule.

    The advantage of having one is also being able to go out of town without the need of asking other people to feed your fish, who may or may not overfeed as well.

    I recommend Eheim’s fish feeder because it has done well by me and it’s also durable.

    Anyway, rest assured that aquarium fish are hardy and don’t really need food every single day.

    You could even cut back on feeding them every other day and they will still be completely fine.

  • Regularly clean parts of the substrate
  • The aquarium’s substrate keeps most of the waste or uneaten food that could potentially fog up the water column.

    The solution is to clean it out regularly, but don’t forget to only do one third of the whole substrate to not disturb the biofilter of beneficial bacteria, as it mainly inhabits surfaces.

    A porous aquarium substrate is the ideal breeding ground of the beneficial nitrifiers.

    For this reason you need to clean portions of it to give the colonies time to regain their numbers in case of losses.

    Another reason to regularly clean the substrate is that a dead fish may remain unnoticed, which is a sure way to shoot the bacterial and ammonia levels up, causing fogginess as a byproduct in the process.

  • Change the water in smaller portions
  • Reducing the portion of the water you change will reduce the chance of a cloudy freshwater fish tank.

    The more dechlorinated nutrient-rich tap water you pour down your aquarium, the more explosively the heterotrophic bacteria will multiply, hazing everything in there.

    With a water change of no more than 10% to 15% this will not happen.

    It will not be enough to cause a dramatic difference in the available organic nutrients in a short period of time.

  • Replace tap water with RO / DI water in water changes
  • Most of us who keep tropical fish in freshwater aquariums use tap water as a source for our water changing duties.

    This water, however, comes with plenty of organics in it.

    Whenever we dechlorinate it, it becomes bacteria-friendly, which allows for the heterotrophs to feast on the organic compounds inside it, multiplying and fogging up the fish tank.

    A common solution here would be to use water that has gone through reverse osmosis and deionization, which has 99% of its impurities removed in these processes.

    Even after dechlorination, the heterotrophic bacteria will have no business in it, because there’s no source of food for them in there.

    This is one of the many advantages of using RO water for water changes in your freshwater aquarium.

    In fact, the sole change to remineralized RO/DI water could significantly decrease the chance of any algae appearance or it could starve out an existing outbreak.

    Now, you could buy RO/DI water from your pet store on a set price per gallon or you can make it on your own at home with an RO/DI filtering system.

    If you decide on getting your own system it will pay for itself in a matter of months, where buying from the fish store will become costly in the long run.

    To make a recommendation, I will say that I haven’t really tested every one out there but I’m particularly fond of this RO/DI system.

    It’s definitely inexpensive given how overpriced the majority of the models can be.

    The one I linked you to is specifically designed for aquarium use and I vouch for its longevity.

  • Trim overgrown or rotting plant matter
  • Yellowing or wilting aquarium plant growth is to be immediately trimmed when noticed.

    It could very well be the cause behind a heavily planted low-tech tank that’s experiencing a misty white water.

    If your tank has lots of aquatic plants, which you haven’t trimmed in a while it’s worth checking if there’s something rotting away in there.

  • Upgrade to a bigger tank
  • …Because you have too much fish already.

    Gradually overstocking a tank can be okay as long as you patiently add up small fish in small numbers.

    However, if you add more than 3 medium-sized fish (relatively to your tank’s size) you are definitely asking for a bacterial bloom to occur.

    Fish waste is, along with uneaten food, the main source of food for heterotrophic bacteria.

    If you’re certain that your tank is not overstocked and you don’t need a larger one, then perhaps, don’t add too many fish at once next time.

  • Introduce a bottom-feeder fish to the aquarium
  • A trusty bottom-feeder could aid you in controlling food waste in the fish tank.

    It will scavenge for leftovers while others are asleep taking care of the organics, before the heterotrophic bacteria have had the chance to overwhelm the aquarium with their presence.

    It’s best if you have one that’s carnivorous and one that’s herbivorous to cover all possible types of sinking food.

    Mind compatibility when choosing these, as bottom-dwelling fish are often territorial among each other, though there are exceptions from the rule.
  • Use a UV water sterilizer regularly
  • UV Lights work by impeding the excessive presence of bacteria in the water column and though it works for clearing a tank it is also an excellent preventative.

    It is most effective on heterotrophs, the ammonia producers.

    Regular treatments with a UV sterilizer are a very helpful tool in making sure their colonies are kept at a manageable level.

    This, in turn, prevents a potential ammonia spike, because there will not be enough heterotrophs to remineralize excess amounts of organics too quickly for the autotrophs to catch up.

    Getting an Ultraviolet sterilizer will also help you maintain a parasite-free water column, which in my book is huge, having experienced a couple of devastating outbreaks.

Final Words

This should give you a good idea into handling the problematic issue of cloudy water in your tank.

Your aquarium is host to much more than just fish, and if they are not cared for then runaway issues such as bacterial blooms of heterotrophic bacteria can indeed occur.

However, preventatives and understanding go a long way to combatting this.

Foggy water in the aquarium is easy to get rid of once you understand what you’re battling.

For the most part, cloudiness should clear itself after a few days as long as nothing major has changed within your tank.

Assess your fish-feeding habits, as excess food and waste are great for ammonia-producing bacteria and bad for everything else.

Introduce preventatives into your system if murky water is a common issue.

If you have any questions at all feel free to ask in the comments below or, perhaps, simply share your story with me.

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