Why is my turtle tank water turning green? (Cause & Fixes)

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Navigating the murky waters of turtle tank maintenance, pet owners often find themselves confronted with a perplexing problem: green water. Despite their best efforts, the once-clear aquariums can quickly transform into algae-infested environments, leaving both turtles and owners alike in a bind. In this comprehensive guide, we dive into the root causes of green water in turtle tanks, uncovering the culprit behind algae blooms, and exploring effective eradication methods.

Aquarium Turtle
Aquarium Turtle

But then why did your turtle tank’s water turn green instead of staying clear as it’s supposed to?

Not only that but, in my experience, you’ll notice that the tank is also becoming smelly.

It’s time to take action and get rid of the problem before it gets completely out of hand.

Your turtle tank’s green water is caused by a form of microscopic green algae, and in this guide, I will show you how to fix that.

Let’s get moving.

Why is my turtle aquarium water turning green?

A form of microscopic green algae called phytoplankton is constantly lurking in the water of any turtle tank.

When the colonies of phytoplankton are under control there is no noticeable difference in the color of the water.

That being said, if your turtle tank’s water is turning green it likely has to do with an algae bloom:

The presence of enough direct light and nutrients in the water of your tank create favorable conditions for its Phytoplankton population to multiply. The Phytoplanktonic algae spores, colored green, start multiplying really fast and fill the water as a result. This event is known as an algae bloom. The water in your turtle tank then becomes a cloudy green color.

In the case of an algae bloom, changing the water of the tank will not help.

That’s because the algae spores will keep multiplying and returning.

In some cases, water changes can even worsen the bloom because one of the possible causes for it could be tap water itself.

Is algae bloom water harmful to a pet turtle?

Green water in itself is not harmful to pet turtles. The blooming algae spores consume a little more oxygen at night in a process known as respiration but that doesn’t affect turtles. Turtles breathe air, unlike fish.

However, the favorable conditions that have led to the water becoming green could hide potential dangers to a turtle. Such environments are rich in nutrients which promotes the growth of not only algae spores, but also some bacteria and other harmful microorganisms.

Moreover, nutrients in turtle tanks with green water are usually the result of too much ammonia in the system. Ammonia in high quantities can be harmful to a pet turtle’s intestines. Turtles can only swallow under water, which means they swallow some of the water too. Here’s a study that explains how ammonia-rich water can be toxic to Red Eared Sliders, a common turtle pet.

So to play it safe, you’d want to get rid of water issues in your turtle tank as soon as you notice them.

What have you done to the water to help green algae spores bloom so fast?

Green murky water in aquarium
Green murky water in aquarium – Image by GreenPlasticDino

There are two main factors that help algae bloom:

  • Direct sunlight. Algae spores are exceptionally good at collecting sunlight and quickly turning it into energy for themselves. Usually, the heat and UVB lamps we mount over the basking area of our tanks are not responsible for the bloom. What can cause one, however, is direct sunlight from a nearby window;
  • Nutrients in the water. Everything that’s not alive in our turtle tanks will eventually end up as food for something else. The nutrients that algae spores can take advantage of include Nitrate, Nitrite, Ammonia, Phosphate, and so on. Unsuspecting imports or a neglected export of these nutrients can result in their abundance in our turtle tank’s water. One non-visual sign of water that’s too rich in nutrients is its foul, sewer-like smell.

Sometimes our tap water contains too much Phosphate and Nitrate and we unknowingly import those to our tank through water changes.

At the beginning of my turtle-keeping journey, I used to meticulously maintain my tanks but they kept getting algae blooms.

Imagine my surprise when I later discovered that the actual culprit behind my problems was my tap water:

In a moment of despair I bought a basic water testing kit for aquariums and tested the water from the tap. It turned out it had nearly 20 ppm (parts per million) of Nitrate and close to 2 ppm of Phosphate.

Upon further research, I’ve found that both of these can contribute to algae issues.

On the other hand, forgetting or neglecting water changes will also result in nutrient-rich turtle water.

If we don’t change the water often enough, the Nitrate in it will build up.

Turtle food leftovers and turtle poop greatly contribute to this.

Author’s note: Nitrate is a byproduct of the Nitrogen cycle that goes on in a closed ecosystem such as an aquarium. If there are no plants in the water to use this byproduct up as food then algae spores will.

Nitrogen cycle in a turtle tank
Nitrogen cycle in a turtle tank

How to get rid of a green algae bloom in your turtle tank water?

Theoretically, to completely fix the green water in your turtle tank you need to:

  1. Eliminate all the green algae spores in the water.
  2. Take preventative measures to stop the spores from coming back.

But how do you completely get rid of something that multiplies by the minute?

Don’t worry – you won’t have to tear down and redo your whole turtle habitat.

There are easier and faster methods than that.

To completely rid your turtle tank of green algae blooms try either of these methods:

1. Block incoming light to the tank for a week.

One thing you could do is to completely black out your turtle tank.

This way you’re limiting the light that reaches the algae spores which could suppress the bloom.

You cover the tank with some sort of cloth or towels and leave it like that for a week.

Blocking the direct sunlight that may hit your tank from nearby windows also helps.

This does not mean, however, that you should limit the daily UVB light your turtle gets. Turtles need their daily basking to remain healthy in the long term.

For this reason, it’s best to relocate your turtle to a temporary tank where it could bask and bathe.

Anyway, the problem with this method is that each time I’ve tried it the algae bloom came back.

Moreover, I’ve found that other turtle keepers report the same low success rate when they’ve blacked out their tanks.

I don’t think it’s worth the extra effort.

Setting up temporary tanks, relocating my turtles back and forth, and still having a low chance of clearing up the green in the water is not a good trade-off in my book.

However, there’s one method I’ve found that virtually guarantees success.

The caveat is that it’s not free.

Nevertheless, the second method is way less costly and time-consuming compared to completely blacking out your turtle tank or relocating it away from the window.

2. Set up a small UV water sterilizer for 10 days.

This is the method that has completely cleared algae blooms from my tanks every time I’ve tried it, without fail.

What I’m talking about here is getting a UV water sterilizer.

A UV sterilizer for aquarium use has the following components:

  • An elongated closed chamber;
  • Its own water pump, which pushes dirty water around a UV light bulb;
  • A light bulb that emits ultraviolet light.

The ultraviolet light irradiates the green algae spores that pass through the chamber of the device and as a result, they mutate.

Mutated algae spores can’t multiply. Eventually all algae spores die out and the water in your turtle tank becomes crystal clear again.

The whole process is completely passive and the installation of the UV sterilizer is a breeze.

Here’s a visual example of a UV sterilizer clearing up the green water in a fish tank:

Clearing a cloudy green water aquarium in eight days
Clearing a cloudy green water aquarium in eight days

Also, if you’re concerned whether this is safe for your pet turtle – don’t be.

Clearing up green water with UV sterilizers is a very safe way of algae spores control in a turtle tank.

The water just passes through the tube containing the UV lamp and gets disinfected while your turtle minds its own business, undisturbed.

This also clears up any foul odors from the water. In fact UV sterilizers are one of the best ways to fight smelly water in turtle tanks.

Once the algae bloom and the smell have completely disappeared, you can move on to employing preventative measures to stop it from coming back.

Also, let’s assume it turns out your tap water is the problem, and you don’t have the means to change the water source or filter it beforehand.

In that case, you can periodically run the UV sterilizer as a preventative measure on its own.

Anyway, there’s a wide variety of UV sterilizers that you can get for the purpose.

Make sure to get one that’s not cheaply built because these don’t last long.

On top of that, look for UV sterilizers with very good sealing to avoid potential leaking issues.

If you don’t have the time to do proper research on these and are okay with ordering online then you can check a good one on Amazon here. I’ve chosen a UV sterilizer that matches all of the criteria from above in the link.

A UV clarifier with a 3-Watt bulb can treat any turtle tank that’s under 20 gallons in less than a week. The 9-Watt version can do the same in tanks that hold no more than 55 gallons of water.

If your tank is larger than that, then I recommend checking my reviews of UV sterilizers for aquariums. This guide has a section with recommendations based on the gallon capacity of a tank, which could save you some time.

However, if you do want to understand UV sterilizers in-depth, I recommend reading the whole review guide.

Author’s note: Clearing up green water with a UV sterilizer also works in fish aquariums that suffer from algae blooms. I’m very happy I’ve discovered this method.

Why is my turtle tank water green?
Why is my turtle tank water green?

How to keep turtle tank water crystal clear in the first place?

I have a separate post that discusses turtle water and how to keep it clean in depth. Aside from the ways to do so, the guide also contains an example calendar for good turtle tank maintenance.

To summarize:

  • Get a strong canister filter and stuff it with what’s known as filter floss media;
  • By using an aquarium siphon, clean uneaten foods and turtle waste which can contribute to green water;
  • Load the tank with some floating plants that turtles are not interested in eating;
  • Add small and agile algae eaters such as Ghost shrimp to the tank (this is for hair algae, and not spores);
  • Change tank water with dechlorinated water;
  • Change or treat the water’s source in advance if it already is nutrient-rich.

Though all of these will keep your turtle aquarium pristine, running a UV sterilizer on the side is one of the best ways to keep green water at bay.

Tell me how things unfolded for your turtle’s habitat in the comments.

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Photo of author


Momchil Boyanov is the Founder and now Senior Editor of AquAnswers. He has over 13+ years of experience in keeping home aquariums as well as providing professional aquarium services. Momchil has had his fair share of adventures in aquarium care. He has made MANY mistakes throughout his fishkeeping journey and thus learned A LOT. Through Aquanswers, Momchil shares knowledge about freshwater and saltwater aquariums with the people within this community.

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