Does Green Aquarium Water Hurt Your Fish?

When my aquarium water suddenly turned green, up to the point where I could hardly see the fish swimming in the aquarium anymore, I was dumbfound. I had no clue how this came to be, and I wondered if my fish were in danger. I quickly did some research, and this is what I found.

Is green water in your aquarium bad for your fish? While green water certainly looks terrible, it is not harmful to your fish. Thousands of single celled algae particles are floating through the fish tank. When the concentration is high enough, your fish can be hardly visible.

So luckily the green water itself is not harmful to your fish, but there are still things you need to be careful with concerning green water. And also, it looks terrible so how can you get rid of it quickly? These are some things which I will quickly go over, so you know all the ins and outs.

What is the cause of green water

The green water in an aquarium can happen rather suddenly. It sometimes is also called an algae-bloom. Within a couple of days your water is completely green and you can only see a couple of inches / centimeters into your tank. But what causes this?

The main thing that plays a big part in green water is light. If you have rather bright aquarium lights, or especially when your tank is (partially) exposed to direct sunlight, your aquarium is vulnerable to green water. This is because the algae thrives with bright light, as they are depended on photosynthesis.

Another thing that is part of the cause is an accumulation of nitrates and phosphates in the water. You might also call this poor water quality or lack of proper regular maintenance.

When the concentration of fertilizers (nitrates/phosphates) is super high, it can happen that the algae suddenly finds traction and takes off, causing the algae-bloom.

Possible treatments for green water

  • Blackout technique
  • Daphnia that will eat the green water (only effective when they do not immediately get eaten by other fish)
  • Use a diatom or micron filter
  • Use a UV sterilizer

There are also a lot of products on the market that you can buy, that claim to treat green water. These in my opinion are not the way to go. Most of the times, the cure will kill more fish than the green water itself. Also, there are other ways to treat green water, which makes it a waste of money.

The blackout technique

Instead of relying solely on chemicals to fix your green water problem, a popular technique is the blackout cure. When you have been dealing with this problem for a while, you know that just doing water changes will not help.

This is the method I recommend the most, because a lot of people have effectively used it to get rid of the algae/green water.

The first step is to fully cover your aquarium in cardboard or blankets to make sure there is no light coming through and reaching your aquarium. Because there is no light, the algae floating in the water will die off.

If you have relatively hardy plants that do well in shady areas, like java fern, cryptocoryne and anubias, the maximum time you can keep your aquarium covered is about 2 weeks. If you have more stem plants that require a lot of light, limit the cover time to 1 week.

Because all plants are more hardy than the free floating algae suspended in your water, they will survive the treatment. The algae will die off.

While doing this, make sure your water is properly aerated and shut off your CO2.

Add daphnia to your tank

The second treatment is to add daphnia to your tank, which will eat the algae particles in the water. They love them! However, make sure you only do this treatment when the fish in your aquarium do not eat all the daphnia straight away.

Diatom / Micron Filter or UV Sterilizer

There are also a couple of mechanical solutions that you can use to fight green water. These pieces of equipment might be new to you so I will go over all 3 briefly to explain what they do.

Diatom / Micron Filter
A diatom filter is a filter that is designed for super fine mechanical filtrations. These diatom filter are very well suitable to use to clean the water after a water change, and they are also recommended to trap the green water algae particles. If you are thinking of getting a diatom filter, know that it is not recommended for long-term everyday use.

UV Sterilizer
This might be the most effective way to actively clean the water to remove any living particles from the water. It uses ultra violet light to kill microscopic organism that are free floating in the aquarium water. It also kills parasites, viruses and bacteria. Note that it both kills the bad and the good bacteria.

If you are thinking of getting a uv sterilizer, I recommend the AA aquarium green killing machine. It has a 4 out of 5 star rating with over 800 reviews on Amazon. Oh and it also has the cheesy quote “say GOODBYE to GREEN and HELLO to CLEAN” in the description, that is probably why it only got 4 starts instead of 5.

How to prevent green water in the future

With these techniques, I hope you now have to knowledge to effectively treat the green water. Let us hope that you can watch your fish again in no time.

Now it is treated, it must be prevented for the future. To do this, keep in mind the following tips.

  • Avoid direct sunlight on your tank
  • Avoid overfeeding your fish, causing a spike in nitrates and phosphates
  • Do not overstock your tank, too many fish will cause too much ammonia
  • Run a UV Sterilizer
  • Do your regular water changes

The first and foremost factor to check, is my tank in direct sunlight at some point during the day. If yes, think about moving the tank if you want to decrease your chance of an algae bloom.

Also, do not overfeed your fish and overstock your tank. Both will add a source of ammonia to your tank that will in its turn cause a spike in nitrates and phosphates.

With an excess of nitrates and phosphates, especially if you let them accumulate, the chance that you will experience green water again rises.

If you really do not want to have any algae floating through your aquarium, you can let a UV sterilizer help you with this. When you keep this running constantly, it will take care of all the floating algae and parasites in your tank. Just remember that it also destroys any good bacteria that are present in your tank.


And finally, do your regular water changes. I know this sometimes is the dull part of the hobby, and that we maybe do not always want to do them, but it is worth it in the end. We are responsible for the environment our fish live in.

Related Questions

Why does my tank glass turn green? Your glass turning green is algae, and it is super common in the aquarium hobby. Especially at the start, when your aquarium has not found its perfect balance yet, algae will haunt you. If will be stuck to your tank, substrate and plants, and the only way to get them off is by manually removing them. Take an old bank card and scrape the algae off the glass carefully. There are also dedicated tools out there to do so.

How to get rid of algae in fish tank naturally? Getting rid of algae, or rather preventing the growth of algae, has to do with a balance in the aquarium. If you have a planted tank and your aquarium is balanced, no excess algae will grow. To remove the algae from the tank naturally, you can try introducing an algae eating animal like nerite snails or otocinclus to your tank.

Do all aquariums have algae? Yes, all aquariums have at least some algae in them. Most of times it is not bad to have algae, but it can become a problem when it grows out of control. Green water and algae on the glass obscures the view of the fish.

Bart Sprenkels

I have been keeping multiple aquariums since I was 18 years old. Just like many of you, I started with two goldfish but quickly learned they were not suitable for aquariums. Later, I switched to a tropical community tank and I also have two pet musk-turtles in a bigger aquarium. You can read more about me here.

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