How to get rid of green water (algae bloom) in an aquarium?


You’ve got your new aquarium started and everything has been going great; your fish seem happy, active and everything seems like smooth sailing from here on out.

Then you notice the aquarium water having a strange green tint to it. You think it’s nothing at first, however, this is far from the truth.

What you’re seeing is the beginning of an algae bloom.

Is this condition harmful to fish and other aquatic inhabitants?

A good aquarium owner needs to know how to stop a bloom fast when it begins to turn the water cloudy, because the more they wait, the longer it takes to resolve the issue.

Both freshwater and saltwater tanks are susceptible to this plague so always be on the lookout for any time your water is not as clear as it should be.

The good news is that this green intruder in your aquarium water can be evicted.

Let’s explore the solutions to this problem together.

What’s causing the water in your aquarium to become green?

Green aquarium water can be very distressing the first time you see it.

However, to be effective in combating this you need to understand what is causing this.

The reason for aquarium water to become green is:

Micro-algae spores, also known as phytoplankton, are the reason behind aquarium water that’s turning green. These spores are always present in a fish tank, but when they bloom the result is a cloudy green tint to the water column. Reducing the available nutrients in the water can suppress the bloom and clear it up.

These algal blooms thrive on the same things that any other plant does.

The main factors here are light and food.

When your aquarium has an excess of either of these, it is when green water starts being an issue.

Tested ways to get rid of green water in your aquarium

Clearing the water up is as simple as improving its biological or mechanical filtration.

That being said, to get rid of green-colored water in your aquarium you can:

  1. Install a UV water sterilizer.
  2. Set up a fine-fiber filter floss media.
  3. Reduce the blue spectrum of your lighting fixture.
  4. Manage nitrates.
  5. Clean the substrate.
  6. Add an aerator to the aquarium.
  7. Lower phosphate levels in the water.

1. Install a UV water sterilizer

Here’s how effective a UV sterilizer can be for clearing the cloudy water in a fish tank:
A UV sterilizer clearing an algae bloom and removing the green color of aquarium water.

By awmbonke

By far the fastest method to clear the green in aquarium water is using a UV sterilizer.

This method works by bombarding the cells of the algae with strong UV radiation, causing mutations directly in their genome.

This, in turn, prevents algae cells from multiplying and employing nutrients efficiently.

In my experience, UV sterilizers are a highly effective way of waging war on the micro-fauna causing trouble in your fish tank.

Another reason why I recommend this method is that you only target the water column, whereas with a chemical treatment you may or may not cause harm to live aquatic plants as well.

Using a UV water clarifier can absolutely eliminate the need for implementing any of the other methods I’ve listed.

It’s what I did back when I needed to because no one is immune to green water in their fish tank.

After my initial purchase, I use the same UV sterilizer over and over on my newer tanks.

I’ve personally had some very pleasing results with AA Aquarium’s 3-Watt UV Sterilizer for smaller tanks (5 to 20 gallons of water).

You can check that unit at both Chewy.com and Amazon.com and compare pricing, etc.

The 9-Watt version of the aforementioned UV Sterilizer will likely work best for clearing the green water in aquariums of 30 to 55 gallons.

Click here to see the 9-Watt Green Killing Machine at Chewy, or click here to check it out on Amazon.

For larger tanks though, I’ve left a link to a guide on UV bulb Wattage in relation to aquarium size, just a couple of sentences below.

Anyway, the aquarium water is usually 100% transparent in 4 days or less, depending on how heavy the bloom was and how dense the green appears to be.

Another additional benefit of this method is working with any type of tank, really, be it freshwater or saltwater.

A thing you may want to be aware of is that the power and time needed for the UV to work its magic varies depending on the size of your aquarium.

During my early days of aquarium keeping, this was the solution that suited me the best.

I woke up one morning with my fish tank’s water almost soupy green. Fearing for the life of my fish, I tried quite a few methods available at my local aquarium shop (pure panic).

Nothing seemed to get rid of all the algae though. The green pea soup color would disappear and then it’d randomly come back.

Finally, I managed to fully clear the water by getting my hands on a UV sterilizer. It arrived in the mail right on time as I was already desperate and at my wit’s end (with my limited knowledge back then).

Anyhow, as far as permanent solutions go, I’d recommend these water sterilizing filters all day long so that others don’t have to go through the headaches I experienced back then.

The UV sterilizer a rather beginner-friendly solution, but you do need to know a couple of things before getting one.

I’ll save you 5 hours of research then – this is a cheat sheet on selecting the best and most effective one for your tank’s size and setup.

Nailing the right UV sterilizer will clear the green from your aquarium’s water in just about 4 days, from my experience.

Give the cheat sheet a skim if you’re into “set it and forget it” solutions, as other methods mentioned below will likely take more than 2 weeks to come into effect.

2. Set up a fine-fiber filter floss media

The idea here is to use a fine filter media to strain your water even further than your standard filtration system allows.

This solution works well as a temporary holdover or in conjunction with other methods.

But, be aware that in the case of free-floating algae it will not be enough.

It works well for making cloudy water more clear, but the green tint in your water will linger as long as the bloom’s still live.

To use filter media simply place it somewhere in your filter and be sure to replace it fairly often.

The right filter floss will be really efficient in catching small particles.

This includes microalgae spores, responsible for making the water in your aquarium green.

However, this exact property means that it will build up with particles fairly quickly depending on how dirty the tank is and will stop being effective once filled up.

Think of floss filtration media like this: it’s a way of polishing up the water in your tank, but not as a way of fixing the issue that caused the dirty water in the first place.

Still, there’s definitely a place for it in aquarium care, especially as a low budget option to hold over for the green water until a permanent sanitizer is installed.

There’s a catch to selecting the right one, and if you play your cards smart you can get it dirt cheap. Here’s a straightforward guide on everything you’ll need to know on that.

3. Reduce the blue spectrum on your aquarium lighting fixture

An interesting solution to at least limit the development of algal blooms and, consequently, green water in your aquarium is to lower the blue light spectrum that is coming from your lighting.

This can be accomplished through the simple means of changing up the type of light you’re using, such as an LED set up that focuses mostly on the red light.

If you already have an LED fixture for your aquarium lighting but it lacks a customizable spectrum then, perhaps, it’s time to consider an upgrade.

Since the research on these can be daunting for the newcomer, I think I should mention that I have a pretty detailed article discussing both LED lights and how the spectrum relates to plant growth and photosynthesis.

Algae thrive in blue light more so than in other spectrums, because it employs these wavelengths best for its photosynthesis.

Other aquatic plants also make use of blue light, but not to the same extent as the algae that caused your fish tank’s water to become green.

This means that by lowering the intensity of blue light diodes (wavelength of 435 to 495 nanometers), you’re limiting the growth of algae while also bolstering the light that the aquatic plants that are actually good for your tank enjoy.

I wouldn’t recommend this as a cure for algae necessarily, but if the lighting was the problem it is an excellent preventative measure that will help the overall health of your aquarium’s water without having to buy any additional equipment.

Look into light spectrums and how they affect plants in aquariums in the article I linked you to above.

It’s an often overlooked aspect of aquarium management, that I rarely see mentioned in online literature, especially when it comes to the phytoplankton blooms, responsible for making the water green.

Note: If your LED fixture has a “night mode” (actinic blue light) which you leave on during the night purely for aesthetic reasons – you should likely consider turning it off.

Removing the actinic blue light from your LED lights’ schedule altogether can help with clearing up the cloudy green water in your fish tank by a large margin.

4. Manage nitrates

If you’re experiencing what seems to be a neverending algal bloom then you’ll need to nip the root cause of this in the bud.

Algae need food to survive, and this comes in the form of high nitrates from decomposing fish food and waste.

An overpopulated tank will need additional maintenance and cleaning to ensure no green micro-algae overtakes.

This is because fish waste produces organic materials, such as ammonia, which are converted to nitrates by nitrifying bacteria your tank, and algae absolutely love nitrates.

Unintuitively, changing the aquarium’s water too often can actually compound imbalance issues.

This is due to a constant microscopic battle that is occurring within the water between zooplankton and phytoplankton.

The former prey on the latter. However, zooplankton tends to be more sensitive to changes in the water balance.

Constantly changing out the water can lead to a decreased zooplankton population and an abundance of phytoplankton which are the main perpetrators in algae blooms.

Beyond this, a common folly of new tank owners is that of overfeeding their fish.

It may seem like a non-issue at first glance but any extra food that isn’t eaten ends up sinking to the bottom of the tank.

There it slowly dissolves thus creating a nutrient-dense environment, which is fundamental for the development of green aquarium water.

A lot of people see this occur in their early days of aquariums and should quickly rectify this issue to ensure a healthy habitat.

This should only be a consideration after you’ve resolved the green water issue, but I think it’s now worth mentioning. Overfeeding can also be avoided with an automatic fish feeder.

I haven’t really done a head to head test to all of these, but that’s because I actually ended up nailing the right one from the first time.

I trusted my gut and went for an already established brand in the fishkeeping market (that would be Eheim) and got myself this unit.

In the link, I’m showing you the exact model of the automatic fish feeder which has been really reliable in helping me dose the right amount of food for my fish.

5. Clean the substrate

This goes off of the last topic a bit but deserves its own attention.

Over time, the substrate at the bottom of your tank becomes its own ecosystem.

This is normal, however, if overfeeding or overpopulation was an issue this ecosystem living within the substrate may be filled with heterotrophic bacteria.

Constantly recurring algal bloom and persistent green water can very well be caused by pockets of heterotrophic bacteria still thriving in the nutrient-dense substrate at the bottom of your aquarium. Let me explain.

Heterotrophic bacteria feed on nutrients such as organic waste. If not maintained well small particles of food and fish waste will settle in the substrate and start degrading.

Heterotrophic bacteria feed on these and turns them into ammonia and CO2. Both of these byproducts are essential for plant and phytoplankton growth.

Essentially, when the substrate is not cleaned well or you don’t have enough plants to take up the abundance of nutrients – microalgal spores will bloom and cause the water to become green.

Note that heterotrophic bacterial bloom often goes hand by hand with phytoplankton blooms.

The former causes the water to become foggy, with a white hue.

This is often the case with new fish tanks right after the nitrogen cycle starts to develop.

This method works best in combination with having a UV sterilizer which in turn will eliminate the free-floating bacteria that feed off the nutrients in the water.

6. Add an aerator to the aquarium

For the processes listed in my previous section, you may want to add an aerator to your aquarium.

This is not a direct solution that will get your aquarium rid of green water, but rather a preventive measure while the balance in the ecosystem is being restored.

Both of these bacteria require huge amounts of oxygen in the water.

Why am I recommending an aerator then, wouldn’t that help them thrive?

Well, guess who also needs oxygen to survive – your fish.

During a bacterial and algae bloom the demands for oxygen rise and your fish are essentially competing with the micro-flora and fauna in the water. This effect is multiplied during night hours.

Plants and algae produce oxygen during the day, turning energy into sugars.

During the night, however, a process called respiration takes place, which employs said sugars for cell food.

You could say that respiration is the opposite of photosynthesis. Turning sugars into cell energy actually uses up oxygen.

All of this means, that if your aquarium already has green water, during night hours your fish may be competing for oxygen with the phytoplankton.

Less hardy fish species may suffocate.

By adding an aerator you make sure there’s enough oxygen for everyone in the fish tank and you’ll have peace of mind while fighting off the green water.

7. Lower phosphate levels in the water.

Algae thrive in phosphate (PO4) rich waters. If your water test shows over 1 ppm of phosphates then you can be sure that it’s at least a great deal of why the water in your fish tank has become green.

To reduce PO4 you should know what’s behind it:

  • Tap water in older cities.
  • Use of carbon filter media without phosphate absorbers
  • Uncleaned filter
  • Uneaten fish food
  • Fish food rich in PO4
  • kH buffers
  • pH buffers
  • Aquarium salts

One too many possible culprits if you ask me. In my experience, however, the most likely reason behind elevated phosphates in the aquarium is tap water.

If you suspect high PO4 levels in turning your aquarium’s water green, then your next course of action should be to test your tap water.

If this scenario gets confirmed then it may be time to consider a different source of water for your fish tank.

Distilled water, purified water, and RO/DI water are all options as long as you remineralize them.

The most cost-efficient method, in the long run, would be getting your own aquarium RO DI system.

The guide I linked you to lists the best units to use in a reef tank, but that’s on purpose because you need pristine water that lacks excess nutrients when looking after aquarium corals.

Each of the RO/DI systems in there will do a fantastic job at ridding your tap water from phosphates and other impurities, like nitrates.

This, in turn, will eliminate the possibility of these impurities cause the water column to become hazy and green.

Don’t forget that remineralization of the filtered water is needed because the purification processes strip both beneficial minerals and harmful particles.

What causes an Algae Bloom in the first place?

We’ve talked a bit about how to deal with the situation once it’s occurred but it’s also beneficial to know exactly what the general root causes are.

Here’s what causes a micro-algae bloom and green water in a fish tank:

Phytoplankton algae blooms are caused by an imbalance of organic materials within a fish tank.

The most common causes of green water are the overfeeding of fish and also fish waste in an overpopulated system. This extra organic material will accumulate in the substrate of the tank where it will decompose leaving extra nutrients for algae causing spores of phytoplankton, which can take advantage of and subsist on this freely.

It should be noted that it is normal and healthy to have some amount of phytoplankton within your system, this won’t cause cloudy or green water.

Generally, they should be kept in check by healthy plants that subsist on the same organic materials as them and predatory zooplankton.

This goes for both freshwater and saltwater tanks, though the breeds of planktons are different.

Is green water damaging to fish and invertebrates?

Though some algal blooms in the wild are definitely toxic to wild aquatic life it is not entirely true for home aquaria.

The answer to this depends on a few factors, and shouldn’t be generalized as the severity of the bloom and the system you’re using play a role.

Whether green water is harmful to aquatic fish is defined when:

Small amounts of green are not harmful to the fish or plants in your aquarium water. It’s a fairly natural thing to have free-floating algae spores crop up from time to time in their natural environments.

However, large algal blooms use up oxygen at night time.

If there is an abundance of free-floating phytoplankton in combination with a heterotrophic bacterial bloom in an aquarium, this can suffocate the fish while their owner is asleep.

In general, it’s good to know that less than 1% of blooms produce toxins.

The problem with keeping a small phytoplankton population in a tank is that you never know exactly how big it can become.

One day it may be a small green cloud and then you wake up to a nasty pea-green soup taking up the resources for your fish and plants.

It’s happened to myself and many other aquarium owners I know.

It should be dealt with very quickly in order to avoid any serious consequences.

How to stop your aquarium’s water from ever going green again?

These should not be used as a cure for algae once it has occurred, but more of a preventative measure to keep a healthy balance in your aquarium for the future.

To prevent your fish tank from going green you can:

  • Run a UV sterilizer for 4 hours every other day
  • Introduce algae eating snails shrimp or fish to your tank
  • Minimize direct sunlight on the fish tank
  • Grow healthy plants in your aquarium that take resources from the algae
  • Raise water flea cultures
  • Treat algae spots carefully with Hydrogen Peroxide or Algicides
  • Remove any existing algae with a scraper

All of these methods will permanently prevent your aquarium water from going green.

Some are more effective than others and I will discuss each below.

1. Run a UV sterilizer for 4 hours every other day

Setting up and running a UV water filter will never allow an imbalance of harmful organisms, not only phytoplankton.

This type of equipment is very popular among freshwater and saltwater fishkeepers alike, because of how effective it is.

It also prevents a number of diseases, ick included.

I left a link to a buyer’s guide earlier in this article in case you’re not sure which one to get and what will be the most compatible with your tank’s size.

2. Introduce algae eating snails, shrimp or fish to your tank

This is another preventative measure that can go well with other algal combatting actions.

These little guys are quite effective at cleaning up any algae that is sticking to the walls of the tank or even hair algae but don’t expect them to do the whole job for you.

If you’re currently dealing with a green bloom, this is not the solution you’re looking for.

They’ll be affected just like anything else in the tank and they should only be added to an already healthy tank or one that has macro-algae issues.

That being said, algae eaters are constantly on the lookout for globules of algae to eat up. Chances are they have a better eye than you for it.

Ramshorn Snails and Bristlenose plecos are popular additions to any freshwater system while tangs (or surgeonfish) are some of the go-to’s for saltwater reef tanks.

They really do a great job spot cleaning the aquarium. This solution is probably the most fun in my eyes, I mean, who doesn’t want more creatures in their ecosystem?

Here’s a list of the most efficient algae-cleaning aquatic critters.

Give it a skim as there are some pretty interesting and cool-looking animals included.

3. Minimize direct sunlight on the fish tank

Sunlight provides plenty of energy to plants, and especially in a closed system such as our beloved fish tanks.

However, algal spores can quickly take advantage of the extra energy and start multiplying aggressively.

This is a VERY common mistake for the new fishkeeper, but it’s also the easiest to fix.

Just place your aquarium nowhere near direct sunlight to avoid the possibility of its water turning green.

Ponds are especially susceptible to green water because of their daily exposure to direct sunlight.

4. Grow Healthy Plants in your Aquarium

Surprisingly, the solution for a lot of tanks is to actually add more plants.

Allelopathy, the idea that certain plants release chemicals that combat other plants including algae, is sometimes given credit for why their addition seems to help so much.

There is some amount of credit to this in studies, but the actual reason this works is much simpler:

By adding beneficial plants to your aquarium they actually take up a lot of the resources, such as excess nitrates, that algae and phytoplankton would use to grow.

Floating live plants grow astonishingly fast and are known as “nutrient sponges” in the hobby. Visit this page to skim through the best selection of these for your freshwater aquarium.

This is, again, not necessarily useful for a currently green fish tank water but it will stop blooms from happening in the first place.

5. Raise water flea cultures

Water fleas prey on excessive phytoplankton in an aquarium.

Having a healthy colony of water fleas will keep the algal spores in check, crushing the possibility of green water in advance.

On top of that, they have another beneficial impact on aquarium eco-systems.

They are super nutritious and most fish have a taste for them. This could significantly reduce the need for feeding in general, once the culture has established itself.

You can find water fleas in most pet stores, where they are commonly known as Daphnia.

Note that growing Daphnia in your fish tank will only be efficient as prevention and not a solution to green water.

If you introduce Daphnia to already imbalanced tank water with a green tint it will take weeks before any visually noticeable effect to take place.

6. Treat Algae Spots Carefully with Hydrogen Peroxide

You don’t want to use too much of this, and you definitely need to be sure not to spray your fish with H202, but it is effective at removing stubborn algae spots on the tank.

A small syringe or sprayer works and can quickly blast apart hard to get algae on the walls or decorations within your tank.

Dosing 1 ml of hydrogen peroxide per 1 gallon of aquarium water is generally considered safe, but if you have some super tender plants such as Japanese moss balls you should lower the dose to 0.5 ml.

Do this only if you know your aquarium plants well.

7. Remove any existing algae with a scraper

This goes hand in hand with the last one but should be taken as a step of its own.

Use a scraper tool to get as many algae off the sides of the tank as possible, it’s ok if this stirs it up some.

Algae is much more resilient when stuck to an object, so anything to remove it from its hold should help with taking back your aquarium.

The green light to do something about it

These methods should take care of just about any problematic algae bloom situation you might find yourself in and put you in a better spot to prevent them in the future.

Following this guide properly will indeed let you get rid of unsightly green water issues in your aquarium, and in a general sense create a healthier ecosystem for your fish and plants to live in.

Remember that algae are always present in the environment, but with the proper balancing of nutrients in your tank and the right tools to get rid of blooms when they occur, it can exist in harmony with the rest of the life you’re taking care of.

If you need more advice you can leave me a comment below and I’ll make sure to quickly respond.

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Mary Ellen Wood

I’m wondering if after using the UV sterilizer I need to use some cleaning bacteria? Does the UV sterilizer kill the good bacteria too?

Momchil

Hi, Mary,

That’s a really good and logical question. I have answered it completely in my article on UV sterilizers which I linked to in this guide.

Regards

Dean

We made the mistake of putting the fish in only hours after we put the water in(tap) then 10 of the15 fish died in a few days. So we put the live fish in a much smaller tank while we tried cleaning the tank again.
We cleaned it thoroughly and the stones then turned the filter on and haven’t had any fish in it. We just started the 3rd week and green water again has started. Heeelp!!