Why does fish tank water turns green

Why Aquarium Water Turns Green

Learn about why fish tank water turns green and how to treat Your fish tank is normal with clear water and happy fishes swimming around the tank width. You admire the beauty before going to the office or before hitting your bed. But when you return back home or wake up in the morning, a shock strikes. Your beautiful crystal clear fish tank has turned like a mini Amazon swamp due to the stale green water. You get pissed off and make a water change. But within a few days, the water again turns green. You take more advanced steps by cleaning the filter or even adding another powerful filter inside the tank. But still, the green nightmare haunts you again. You then start to wonder why the fish tank water turns green. Clear Fish tank water turns green due to a sudden outburst of suspended algae called Phytoplankton. They float as millions inside the tank water and bloom rapidly. This makes the crystal clear fish tank water turn green within a few days. There are many algae that grow on the walls and bottom of tanks. They grow slowly and can be easily controlled. Phytoplankton is floating algae. This makes it difficult to handle and eliminate. When not controlled quickly, they will grow in a rapid phase covering the whole upper surface of the tank. This will significantly reduce the light entering the tank and the dissolved oxygen content of water. But not all phytoplankton are harmful. There are few phytoplankton types that are grown in saltwater aquariums and freshwater aquariums as a food source. Infusoria is a type of phytoplankton used in freshwater aquariums as natural food for the inmates. In this blog, we will Explain Why the water turns green and how to prevent the fish tank water from turning green. In case your fish tank water has turned green, you can treat it by using the methods mentioned in the blog. So, Sit Back, Relax and Enjoy the Read.

Why Does Fish Tank Water Turn green?

Aquarium water turns green
Fish tank water turns green mainly due to the Phytoplankton outburst. But what causes the phytoplankton outburst in your aquariums? There are few factors that encourage the rapid growth of phytoplankton inside the tank. They are 

Excessive Lights

When plants are exposed to excessive light with enough nutrition, they tend to grow faster than normal. Green Algae are also like plants. When algae are exposed to good sunlight, they will colonize rapidly. When a tank containing phytoplankton is exposed to an adequate amount of direct sunlight with enough nutrients, it will cause an algae outburst. This is called Blooming inside the tank. These light sources can range from direct sunlight falling through windows or strong fish tank lights glowing for longer hours.

Irregular tank cleaning and Maintenance

When the tank is not cleaned and maintained regularly, it will attract unnecessary dirt and algal growth inside the tank. Improper tank cleaning is one of the major reasons for Aquarium water turning green which results in algal outbursts inside the tanks. When tanks are not cleaned regularly, it leads to an increase in nitrate and phosphate concentration in the water. This is due to the fish excretion waste getting accumulated for a longer period. Phytoplankton loves to feast on phosphate and nitrate in water. This added with a proper light source will cause algal bloom.


Overfeeding your fish will make them lazy and unhealthy. These fishes will leave the food particles as it is when they are full. These pellets and food particles then become food for these algae to feed and grow on. Also feeding food that is incompatible with your fish will also make a mess. These are the reasons for fish tank water turning green. But, how to treat the fish tank water if it has turned green?

How to Treat Green Water in Fish Tanks?

When an algal outbreak happens, it turns the water green due to algal bloom. But it can be treated by using a few methods they are 

UV Treatment

Ultraviolet rays can be highly effective in controlling algal growth inside the tank. So, UV Sterilization of your fish tank will easily help you get rid of the green water in the aquarium.
But don’t worry! Fishes aren’t affected by ultraviolet rays. When the UV radiations are passed inside the tank, it destroys the excessive algal growth and turns your green water into crystal clear water.
UV Treatment also controls the growth of other harmful microorganisms. UV Sterilizers are more affordable than ever before. Better purchase one if your fish tank water turns green frequently.

Cover Tank Temporarily

Excessive light is one of the major reasons for a sudden algal bloom. So, you can try to cover your fish tank completely. This will cut the light from passing inside the tank. When light doesn’t pass inside the tank, Algae will struggle to survive. Cover your fish tank with a dark cloth or Black plastic completely spotless. Make sure to keep it covered for at least a day. Your fish won’t be affected if there is no light inside the tank for a day or two. If your fish can’t stand darkness even for a few hours, you can change them to another tank and perform this technique. You can keep the tank covered for a maximum of three days. Make sure to feed them regularly and close the covers immediately after feeding. Switch off the fish tank lights. But make sure the fish tank filter and heater are running throughout this process. If the water doesn’t turn back normal, consider a complete UV Treatment by UV Sterilization.

Diatomaceous Filters

Diatom filters (Diatomaceous filters) are made from a hard substance called Diatomaceous earth. These are found in deep parts of the ocean. Diatomaceous earth lives by feeding on algae and similar organisms in the ocean. So, when they are introduced in your tank, they will feed on unwanted algal growth and control it efficiently. It is one of the oldest methods followed by fish tank experts and experienced fish keepers. Diatomaceous filters also help to maintain crystal clear water in fish tanks. Consider adding these filters in your tank if you aren’t having one set up inside your aquarium.

Chemical Treatment

Algae removers and other fish tank chemicals can be used to control the algal bloom. But remember. They work by targeting and destroying Phytoplankton and other algae that grow inside the tank. Make sure to remove your fish, decorations, plant materials, and gravel before performing a chemical treatment. It is better to change the water completely rather than treating it with chemicals. Once when water is changed, check water quality and perform UV Treatment. It is better to use RO water after an algal attack in your aquarium.

Pro Tip
Chemicals must always be your last way to go in anything when it comes to the fish tank and fish tank maintenance. 

This is because fishes are delicate creatures as they can’t withstand harsh chemicals in tank water. Foreign bodies like chemicals will cause stress among the inmates. It would lead to diseases and at times even the death of the poor little ones. This was regarding green water treatment. Let us move on. How to control algal growth and tank water turning green in future?

How to Stop Fish Tank Water Turning green?

Negligence and improper maintenance are major reasons for an algal outbreak inside the tank which could result in Aquarium water turning green. But still, it can easily be controlled by following the steps mentioned 

Regular and Proper Tank Maintenance

Regular tank maintenance makes your tank atmosphere clean, neat, and wonderful to watch. It also reduces stress among fish and improves overall health. It is mandatory to clean the tank at least once a month. Perform 10 to 20% water change every week. This will keep your tank water crystal clear and impurity-free. Make sure to use a tank filter to remove waste products periodically. Making frequent water changes will reduce nitrate and phosphate deposition in water. This makes it difficult for the phytoplankton to grow. Rinse and wash your tank filter regularly and change the filter’s activated carbon and chemical contents once every month. You can check our blog on Fish Tank Cleaning where we had provided a detailed guide on how to clean and maintain your aquarium effectively.

Proper Tank Lighting

Always try to avoid direct sunlight falling over the tank surface. Keep your fish tanks away from windows to avoid direct sunlight exposure. You can use shades or dark films to reduce the sunlight intensity falling over your tank. But always try to avoid direct sunlight. If your tanks are kept in a brighter room, consider turning off the tank light during day times. Aquariums with plants require more light for a longer duration than aquariums without any live plants inside. Light is highly essential for these plants to survive and perform photosynthesis. Planted aquariums can be lit for 12 hours straight and non-planted aquariums will be sufficient with 6 hours or less light time. Make sure to avoid over lighting and use stronger lights than required by your fishes and tank dimensions.

Adequate feeding

Study how much food is required for your fish and feed accordingly. Overfeeding will cause them to release more nitrate and phosphate through waste materials. This will also make them lazy and sick. Make sure to feed according to your fish diet and length. Larger fishes need more food and Nano fishes will be full of less food. Make sure to add food that can be completely eaten within 2 minutes by your fish. Feed once or twice per day. This will make fish less stressed and reduce food waste. You can also skip a day per week to maintain a healthy tank.

Perform Water Tests

Check tank water quality to ensure the water is free from excessive nitrate and phosphate. Tap waters will usually have less nitrate and phosphate. But still, it is better to check for Nitrate and phosphate levels. Also check for pH, temperature, and dissolved chemical contents to maintain a clean and clear Tank. If your tap water is dirty or contaminated, you can go for purified water. Using RO Water in tanks will be a great choice if you don’t have access to clean tap water.

Use Living Plants

Green plants will compete with algae as both require almost the same nutrients and gases. Live plants are more capable of taking out most of these resources for their survival and growth. This will make it difficult for Phytoplankton and other algae to burst inside the tank. Also, fill the tank a little above ¾ of its total capacity. This will make sure the tank is well aerated. While adding fertilizers for the plants inside the tank, make sure to add the exact quantity required. Excessive fertilizer addition in the tank will easily cause algal outbursts.

Final Words for fish keepers

Fish keepers must always remember that a clean and neat tank is less prone to any problem. Make sure to clean your tank every month and perform a 20% water change every week. This will keep your tank crystal clear and your fishes happy and healthy. So, that was all about Green Fish Tank Water and its Treatment. We hope this blog helped you with Fish Tank Green Algal Bloom and how to stop Fish tank water from turning green. We always welcome your suggestions. Comment us down below your experience with fish water turning green and how you came over it. Feel free to mention things that are to be changed and improved in our blogs and website as we are learners for life! If you liked this post do share it with your friends and family. Cause sharing is caring. See ya! Until then, keep growing your aquarium

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