Green Aquarium Water – Causes and Treatments

When fish breeders and hobbyists set up an aquarium, their desire and expectation is to have their finned friend swimming in sparkling clear water as their display their vibrant colors and active swimming habits.

However, over time, most people that keep fish will tell you that green and mushy water is a problem they have to deal with from time to time.

Green water in the aquarium can be frightening for aquarists, especially beginners since they may not know how to deal with it.

Green water in the aquarium can be caused by a number of reasons. It is important to understand what is causing the green water in your aquarium so that you know how to deal with it.

Green Aquarium Water

Green Aquarium Water

Causes of Green Water in the Aquarium

Green aquarium water can be caused by multiple factors. Below you can learn about the causes of green water in new or old fish tanks:


Algae are single celled plants and a type of aquatic plant. There are over 8,000 species of algae on planet earth, such as filamentous algae or black beard algae.

Algae are green in colour and will cause water in your aquarium to turn green too. Like any other plants, algae is facilitated by light in the process of photosynthesis that promotes its growth.

Most aquariums, even those kept spotless clean, have some form algae from time to time. Algae may be seen suspended in the aquarium or attached on the aquarium’s walls or the aquarium’s ornaments.

Some algae in the aquarium is not harmful. It can be a source of food for some of your aquatic beings. However, the problem occurs when the algae grows out of control causing the water to turn opaque and green that it is hard to see the fish in the aquarium.

Excess light

Excess light in the aquarium will not particularly cause the water to turn green in your aquarium. However, excess light, especially direct sunlight, aids water plants such as algae to undergo the process of photosynthesis.

The energy form the light causes the algae to reproduce faster. When algae reproduces too fast, they grow out of control leading to many suspended algae in the water and causing the water to turn into a bright green broth. The green color can get so dense that it makes the water opaque, making it difficult to see through the water.


A fish’s stomach is small. It is smaller than the size of its eye. Most aquarists are advised to feed their fish a pinch of food that is equal to the size of its eye, twice or thrice in a day.

You should only feed your fish food that it will finish within two or three minutes. Food that is left uneaten in the aquarium becomes nutrients that promote growth of algae.

The eaten food will also become waste, causing the aquarium to turn into cloudy water.

Keeping too many fish in the aquarium

Follow the recommended number of fish in the size of your aquarium, for instance, there are a number of fish that should be kept in a 5o gallons water tank, depending on the species of the fish or its size.

Keeping too many fish in your aquarium will lead to excess organic water in the aquarium. Organic waste causes water to turn into a thick soup that is cloudy in nature.

The organic waste will also act as nutrients for the aquatic plant such as algae that are responsible for the green color in water.

Dissolved components

Green and cloudy water in the aquarium can also be caused by dissolved components such as metals and phosphates in the water.

You can test for dissolved components in the water by checking your aquarium’s PH level. If it high or alkaline, then most probably there are many dissolved metals and phosphates in the aquarium.

An alkaline water tank will also cause the aquatic plants in your aquarium to die. The dead matter, that will then start to decay, could be the reason why your aquarium has green and cloudy water.

Leaving the aquarium water for too long

It is advisable to perform water changes in your aquarium frequently. 25% water change every three weeks will ensure all algae, dead matter, dissolved components and organic waste is removed in the water.

On the other hand, leaving the water in your aquarium to go for too long, will only mean the water has high levels of nutrients that may lead to a faster development of algae in the aquarium.

Dirty water will turn cloudy and opaque making the aquarium an unsafe environment for your fish to live in.

How to Treat Green Water in Your Aquarium?

How to Treat Green Water in Your Aquarium?

How to Treat Green Water in Your Aquarium?

So, now that you know the causes of green aquarium water, let’s see how to fix it. Below are some quick tips on how to get rid of green water in your fish tank:

Block out direct sunlight

Once you realize your aquarium is mass of thick green water, the first step is to remove your aquarium from direct sunlight.

As mentioned earlier, light facilitates the process of photosynthesis causing the algae to develop even faster. To provide light for your aquarium, change from direct sunlight to supplemental aquarium lighting.

Change the water in the aquarium

Algae is an aquatic plant that can freely float in water. To treat your aquarium and get rid of the green water, you need to change the water immediately.

You can change 25% of the water in the aquarium, but if the algae has spread too much, consider moving your fish to a temporary water tank as you change the water.

Clean water tank’s surfaces

After moving your fish to a different tank, you should also consider cleaning the tank’s surfaces. Algae will not only float in water but also attach on the aquarium’s walls gravel and ornaments.

Clean the tank before you refill the aquarium with water in order to restore the fish.

Add algae eater in the aquarium

Algae is not entirely bad for an aquarium. It is a source of food for some fish. To treat algae in your aquarium, consider adding algae eaters such as shrimp, daphnia and snails.

These water creatures will feed on the algae in the tank, then in turn they will be a source of food for your fish.

Reduce the number of fish in your aquarium

Organic waste will lead to increased development and growth of algae in the tank. To treat algae in your aquarium, reduce the number of fish in your tank. If not, then get a larger tank that will comfortably support the number of fish you want to keep in your aquarium.

How to Prevent Green Aquarium Water?

As it is with any condition, prevention is always better than cure. Below are tips that will help you to prevent green water and maintain crystal clear water in the aquarium for you to enjoy the view of your finned friend.

Use recommended water for your aquarium

Many tap water contain high concentrations of phosphates, nitrates and other minerals. These minerals support and facilitate faster development and growth of algae.

Therefore, even with constant water changes, if you use tap water, the aquarium will still have algae that causes green water.

Use recommended aquarium water, bought from pet store, or use a de-chlorinator to treat tap water before using it in your aquarium.

Maintain good lighting

Use supplemental lighting for your aquarium rather than direct sunlight. Have the lights on for the recommended hours according to the species of the fish that you keep.

For instance, if you keep a tropical fish, it is recommended to have 12 hours of lighting, some other species only require 8 hours.

Provide the light only for the hours that is needed and switch it off to limit the period of time that photosynthesis occurs in the tank.

Perform regular water changes

Algae growth will be facilitated by organic waste and minerals in the water that accumulate over time. To ensure the water is clean, perform a regular water change.

You can opt for the standard three weeks of 25% water change.

Perform regular aquarium cleaning

Apart from regular water changes, you should also consider a thorough aquarium cleaning from time to time. This will help to remove algae that has accumulated on the walls, decorations and gravel in the tank.

Feed your fish the right food proportion

You can also prevent green water in the aquarium by ensuring there are no leftover foods in the tank. Feed your fish food that it will finish. In addition, avoid feeding your fish too many times in a day.

Two or three times of small sized food portions will be enough. Leftover food will not only facilitate algae growth in the tank, but also cause the water to be cloudy and slimy.

Install water filter

A water filter will help to clean the water in the aquarium. This will help to prevent algae development in the tank.

Wrapping Up

Although green water in the aquarium may appear dreadful, it is not toxic for the fish. But who wants to keep an aquarium in their house with a cloudy and green pigment?

The secret to prevent algae, which causes the green color in the aquarium, is to keep the tank clean and perform regular water changes.

Early treatment of algae growth will prevent more serious problems in the tank.

Fishkeeping   Updated: November 5, 2020
avatar Hello, my name is Fabian, and I am the Chief Editor at Aquarium Nexus. I have over 20 years of experience in keeping and breeding fish. The aquarium hobby brings me immense joy, and I take great pleasure in sharing my experiences with others.

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