Common causes why fish tank water turns green include too much light, nutritional imbalance, and poor maintenance. The transformation of your aquarium water to pea soup can happen in just a few days with one or more of the causes taking effect.

goldfish and green algae on aquarium glass

Initially, some people might think the green aquarium water is caused by a lack of water change. But, what do you do if you see green water a few days after changing your fish tank water?

The factors mentioned above encourage free-floating algae bloom, which causes the population of green water algae. This specie of green water algae is called phytoplankton. They differ from other kinds of algae in the sense that they don’t grow on the aquarium glass or the objects inside, they float and reproduce by the billions. This excess algae growth within a short period is referred to as algae bloom.

This algae growth is harmless to your aquarium fish and other objects, but they make your aquarium water green and in extreme cases, can prevent aquatic plants from receiving direct sunlight.

In some cases, the green algae can be beneficial, especially for saltwater reef tanks because they can serve as plant feed to corals. However, aquarium owners interested in preventing green aquarium water should first become familiar with the common causes of the growth of green algae in your water to get rid of them.

Causes of Green Aquarium Water

Too Much Light

Firstly, algae are live plants and plants love light. For this reason, excessive light in the aquarium can trigger these live plants to grow rapidly. Too much aquarium light is the result of placing your fish tank or planted tank close to a sunny window.

It can also be the result of leaving your tank light on longer than necessary or using aquarium lights that are too powerful for your tank.

Low Maintenance Levels

When you fail to have a regular maintenance routine, it will gradually result in poor water quality. This can make your water a perfect breeding ground for algal growth over time.

We can’t see nutrients with our naked eye, so we won’t know that our water condition is bad until we see the occurrence of algae congestion or your fish getting sick.

Nutritional Imbalance

Plants require nutrients to grow and for most algae plants particularly unicellular algae, the primary nutrients needed are nitrates and phosphates. These nutrients are sourced from fish waste or fish feed. However, the same nutrients can be seen in tap water as well.

The imbalance of nitrate and phosphate levels can be caused by overfeeding your fish because it will result in leftover food or too much waste. Alternatively, the presence of too many fish can also trigger an imbalance of phosphate levels in your water.

When there are too many fishes for your filtration system capacity or your tank size, it will build up nutrients. Also, when your tap water is laden with nutrients and you use it to change your tank water, clear green aquarium water will be the result.

How to Prevent Green Water?

Knowing the causes of greenish water in fish or planted aquariums can be essential to preventing them. Do the following to prevent the occurrence of green water in your fish or planted tanks:

Suitable Lighting

aquarium in bedroom

Don’t directly expose your tanks to light. You can install shades or background material. The reason is to reduce your tank’s exposure to sunlight. In an alternate setting, if your tank is in a bright room, the abundant light can produce energy for algae to grow.

Once you notice your water turning green, switch off the light. Once your water clears, you can increase the light duration gradually.

If you’re nurturing live plants, you don’t need strong light. We recommend the installation of a timer to control the duration of light and achieve a sustainable day and night cycle.

For planted aquariums, a minimum of eight hours of light and a maximum of twelve hours will do. On the other hand, fish aquariums can do well with a maximum of 6 hours

Proper Tank Maintenance

Changing your tank water regularly is great for the maintenance of a healthy aquarium and the prevention of algae congestion. When you change your tank water, you get rid of elements that trigger the occurrence of greenish water.

Furthermore, ensure the freshwater you’re adding to your tank is free of nutrients such as nitrate and phosphate. Also, change the hang-on filter cartridges every month and regularly service your canister filters.

For mechanical or chemical filter media, you can clean or replace them every 4-6 weeks.

Aquarium Water Testing

We recommend getting a water testing kit for your aquarium to test its nitrate and phosphate levels. You can also use it to test your tap water before you channel it to your tank.

Feed or Stock Properly

Avoid overfeeding your fish or overstocking your fish tank. The reason is both will inevitably lead to enhanced phosphate and nitrate levels which algae love and thrive on.

Appropriate Dosing

When applying liquid fertilizers for your plants, we recommend putting less instead of more, particularly if you don’t use carbon dioxide or there are few plants.

Algae are plants as well and excess application of fertilizer can trigger explosive growth and make your water turn green.

What to Do When Your Water Turns Green?

Install a UV Sterilizer

uv light fish tank

UV sterilizer is the most effective and convenient way to tackle the green water problem in your aquarium. As water flows through the UV chamber and UV light shines on your tank water, it eliminates suspended algae and microbes such as bacteria and viruses.

After a few days of using the UV filter, your tank will have crystal clear water. An extra benefit of the UV sterilizer is that it’s safe for your fish, plants, and invertebrates. Moreover, they have become more affordable and are easy to install recently.

Temporarily Eliminate Light

As we mentioned earlier, excess light triggers the explosive growth of algae in your aquarium. Following that logic, getting rid of the light temporarily can prevent algae outbreaks.

We recommend switching off your aquarium light and wrapping your tank with materials that can block light. You can remove the covering to feed your fish every day before covering the tank again.

This solution doesn’t directly tackle the cause of green water in your tank, but it helps your water to stay clear. There are cases where a stronger solution may be recommended if you don’t see the results you desire within 3 days.

Diatom Filter

Diatom filters are kind of obsolete and expensive, but they are efficient in tackling this problem. Diatomaceous earth makes up its filter media, it traps even the tiniest particles of algae and other microbes.

Alternatively, you can fit these filters with a regular filter media if you want to use them for other kinds of filtration too.

Biological Means

There is a specie of fish known as algae eaters. They can feed on the algae in your tank and eliminate the issue of green water.

Chemical Treatment

Several chemicals can effectively tackle green water algae growth. However, they are only to be used as a last resort. Chemical treatments don’t address the source of the problem, and it’s not easy to predict their effects on your plants, fish, and water chemistry.

Despite their apparent shortcomings, they are quite effective against several species of algae, including the species known for causing green water in aquariums.

If you opt for chemicals in the treatment of green water in your tank, ensure you remove chemical filter media such as activated carbon from your filter housing. Also, ensure that there’s good water circulation in your tank and the dosage of these chemicals is according to the net water quantity.

About the Author

Lucas Greer

Lucas vs. Wild - Lucas is a true nature lover and survivalist. When he's not teaching science at school, he can be found in nature, hiking, climbing, camping, and rafting. He knows all the tricks and DIYs for making unclean water drinkable with simple means in an emergency. At school, his students love him for his exciting water filtration projects.

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