Why Is My Fish Tank Water Green (And How to Fix It)

If you have green fish tank water, you’re not alone. It’s very common among aquarium owners for fish tank water to turn green. Greenish water can be caused by a number of factors that result in an algae bloom.

An Algae bloom occurs when there is an overgrowth of algae in the tank. Green water is caused by a free-floating algae bloom called phytoplankton. This form of suspended algae can bloom rapidly when light conditions and nutrients are favorable, and this rapid growth can turn your fish tank water green and cloudy, sometimes looking like pea soup.

Why Is My Fish Tank Water Green

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The cell structure of phytoplankton is made up of two distinct types of organisms:

  1. Protists, which are complex eukaryotic cells, similar to protozoans, make up the larger group. These forms include diatoms (free-floating green water algae). They can reproduce rapidly and create large blooms in response to changing light and temperature and increasing nutrients such as nitrogen, iron, and phosphate.
  2. Cyanobacteria, also known as blue-green algae. These are photosynthetic bacteria that live in both freshwater and marine environments. Cyanobacteria can form dense mats on the surface of ponds and lakes, often called scums.

If your fish tank water is green and smelly, and you would like to know how to prevent it, you have come to the right place.

This short article will answer the question, “why is my fish tank water green?” or any other question regarding green aquarium water. I will also provide some solutions to prevent it from happening in the future.

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Why Is My Fish Tank Cloudy After Wa...
Why Does My Fish Tank Turn Green So Fast

Your fish tank water can turn green quickly with the right conditions, sometimes in only a few hours. So what conditions can cause this sudden free-floating algae bloom?

Algae are living organisms that act much like all aquatic plants in the way they feed and grow. Algae harvests energy through light using photosynthesis, and it feeds on the same nutrients, which are the nitrates and phosphates in the water. When light and nutrients are abundant, an algae bloom can occur and develop rapidly.

Why Does My Fish Tank Turn Green So Fast
Sunlight is a major cause of green algae, so you should keep your tank away from windows and direct sunlight.

The list below is some of the reasons why algae may bloom:

  • Keeping the aquarium light on for too long – A fish tank requires a natural light cycle similar to what is experienced in nature. Fish don’t need light at night, so you should only keep the aquarium light on for 8-12 hours per day.
  • Strong sunlight – If your tank is near a window, it may be penetrated by strong direct sunlight during the day. These intense UV light rays will have a powerful effect on any algae existing in your tank, causing a sudden growth spurt. Keeping your tank close to sunlight will also warm the tank, and warmer temperatures will increase algae growth rates.
  • Warm Tank Water – If your tank water is too warm, it will hold less dissolved oxygen and increase the amount of carbon dioxide that plants and algae feed on.
  • Overfeeding – Overfeeding your fish can cause algae blooms for two reasons. First, uneaten food will decompose, increasing ammonia levels, which in turn increases nitrate and phosphate levels in the water for algae to feed on. Second, when fish are overfed, they produce more waste, adding nutrients to the water that algae use to grow.
  • Too many fish in your tank – Too many fish will cause too much fish waste, which will lead to high levels of nitrates in your aquarium water, which will feed your algae, causing your water to turn green.
  • Water changes – If you’ve ever asked the question, “why is my fish tank cloudy after a water change?” it can be down to a bacterial bloom, algae bloom, or disturbed sediment. Algae blooms can be caused when suspended algae feed on rotting food particles that have been released from the substrate

These are the main reasons why algae can suddenly grow and turn your fish tank water green so fast.

What Causes Green Algae On Aquarium Glass

If you have green algae on your aquarium glass, the most likely cause is that you have your tank too close to a window, causing strong sunlight to penetrate the tank. Algae will move to the closest source of light to grow, which will be on the glass.

The best way to prevent this is to move your aquarium away from the window or shade the tank from any excessive light sources.

Excessive algae in your tank may start to attach to the glass for no apparent reason. As the algae settle, it will attach to almost anything, including other plants, decorations, and rocks. Free-floating algae will usually wipe off quite easily, however, different types of algae may bind more firmly.

You can use vinegar to clean difficult-to-remove algae from glass, which you can read about here: How To Clean A Fish Tank With Vinegar, and don’t forget to clean your tank decorations before the algae become stuck.

Live aquarium plants can be a little more difficult to clean, however, if algae are left, it can begin to suffocate your plants. Find out How To Clean Aquarium Plants.

Is Green Water Bad For Fish

Many people ask, “will cloudy water hurt my fish?” Green cloudy water is caused by algae and is not harmful to fish. In fact, many fish will feed on the algae. The main problem with green water is that it can block out light, make photosynthesis difficult, and affect your aquarium plant’s growth.

Algae can also cause problems with fish tank oxygen levels because algae will compete with plants and fish for dissolved oxygen. Although plants and algae produce oxygen, this is during daylight hours when they consume carbon dioxide. During the night, plants and algae will consume oxygen, leaving an oxygen-deficient environment for your fish.

Fish will often swim at the surface when they need more oxygen, so if you notice your fish swimming at the top of the tank at night, it is likely that the combination of plants and algae is causing an oxygen deficiency in your tank.

Although the oxygen consumption of algae is usually far less than it produces during the day, some situations like shade and cloudiness can reduce light and can impact the overall production of oxygen throughout the day.

Below is a Youtube video that comprehensively covers the problem of green water, listing its causes, whether it is harmful, and some ways to fix it.

How To Fix Green Aquarium Water

How To Clear Green Aquarium Water

Now that you understand the causes of green algae, let’s look at some valuable ways to clear your green water problem.

There are a few things you can do to clear green aquarium water:

  • Increase Water Movement – Algae thrives in still water, so increasing the water movement in your fish tank will make it harder for algae to grow. You can do this by installing a powerhead or an aquarium pump.
  • Install a UV Sterilizer – A UV sterilizer will help to kill algae and bacteria in your tank.
  • Change the Water – This is probably the most effective way to get rid of green water, but it is also the most time-consuming. You will need to do a partial water change of at least 50%.
  • Use Chemical Filters – You can use chemical filters to remove algae from your water. The most common chemical filters are activated carbon and phosphate removers.
  • Use Algae Eaters – Some fish, like plecos, will eat algae. These fish can help to keep your tank clean and clear.
  • Temporarily turn off the tank light – Plants love light, and if algae have no light, they will quickly die off. Your green water problem should clear much quicker by turning off your fish tank light for just a few days.
  • Add a chemical treatment – You will usually find a chemical treatment for most fish tank conditions, and algae are no exception. If you don’t mind adding chemicals to your tank, there are plenty to choose from, and you will find a wider variety of chemical treatments from online pet stores. Some of the brands and treatments I can recommend are API Algaefix, Microbe-Lift Algaway, and Fritz Algae Clean Out. (Click links to compare product prices)

Using these steps should quickly reduce your green water algae, and you will have crystal clear water in no time.

If you want some extra tips to get your water crystal clear, check out my article: How To Get Fish Tank Water Crystal Clear (It’s Easy!)

I also have an article with steps on how to clean aquarium gravel algae which you can find here: How To Clean Fish Tank Gravel Without A Vacuum (In 4 Steps).

How Do I Stop My Fish Tank Going Green

To stop your fish tank from going green due to rapid algae blooms, you have to remove or reduce anything that causes algae to thrive.

Preventing green aquarium water can be accomplished by using the following steps:

  • Don’t overfeed your fish – Fish should only be fed whatever they can eat within a few minutes, as any more will often sit in the substrate where it will rot away. You can also “fast your fish” by not feeding them for 1 or 2 days a week which will not harm them, and they will spend these days looking for food, allowing their digestive systems to rest.
  • Remove uneaten food from the tank – If there is uneaten food in the tank, you should remove it before it rots and increases nitrate and phosphate levels for algae to feed on. In planted tanks, nitrates and phosphates will be required to feed your aquarium plants, but they will absorb these nutrients much more efficiently than algae, so reducing the level of waste to an acceptable level will be fine. Using a gravel siphon will pick up most of the food waste in your aquarium, and make sure to clean around your plants and decorations where food waste can also hide.
  • Reduce lighting time – Light is the primary energy source for algae, derived through photosynthesis. Reducing light will reduce the development of algae. Fish tanks only need 8 to 12 hours of light per day, so why not use a timer switch to alter your lighting period automatically.
  • Move your aquarium away from windows or other strong light sources – Strong sunlight is one of the biggest causes of excess algae growth, so move your tank into an area of neutral light, or block light with a piece of card.
  • Add some algae-eating fish – Certain fish crave algae and spend their day hunting it down and devouring it. Algae eaters like plecos will get on with most other fish and are a great addition to any tank. Many algae eaters will stop algae from covering your aquarium glass.

Doing these things will make it much harder for algae to grow out of control and turn your tank water green.

Wrap Up

Most aquariums will suffer from algae blooms from time to time, and they are not harmful to fish, so there is no need to worry if you have green water. By following the steps in this guide, you will have crystal clear water in no time.

Maintaining crystal clear water is also relatively easy when you understand the leading causes of green water algae. The algae themselves are just organisms that require food and light, so reducing these will cause the algae to die.

Regular water changes will go a long way to reducing any potential problems within an aquarium, and algae are no exception. Weekly or two weekly 20-30% water changes will help keep waste levels down and remove many of the nutrients from the water that algae thrive on.

Thanks for reading! I hope this has helped you understand why your fish tank water might turn green and how you can fix it.

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Jon O'Connell

I have kept both marine and freshwater fish and set up almost 100 aquariums. Although I am no longer doing it professionally, I hope I can help others to enjoy keeping happy, healthy fish by sharing my many years of experience.

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