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why do i have green fish tank water?

Why Do I Have Green Fish Tank Water?

If you have tropical fish, you will know that your fish tank water is supposed to be colourless and clear, but sometimes it can become tinted or cloudy. Your fish tank water turning green is worrying, but does not necessarily indicate a problem. However, you do need to investigate and correct any issues to ensure your fish are safe.

Your fish tank water is green due to the growth of microscopic algae called phytoplankton. The particles form in the water and is known as an algae bloom. The more it multiplies, the greener the water gets. It is common for the algae to grow in a newly installed tank six weeks after setting it up. 

This article will find out the causes of green algae and ways to remove it and keep it out of your fish tank.

Why Does Green Algae Grow in My Fish Tank?

green fish tank water
very, very green.

If your fish tank water appears green, it is likely due to an overgrowth of algae in the water. Algae are single-celled or multicellular organisms that thrive in aquatic environments and require light, nutrients, and carbon dioxide to grow. Here are some common reasons why your fish tank water may be turning green:

  1. Too much light
  2. Excess nutrients
  3. Poor water conditions
  4. Bacterial bloom

To remedy green water in your fish tank, you can try several things, such as reducing nutrient levels in the water through regular water changes, reducing the amount of light the aquarium receives, and improving water circulation with a good filtration system. Additionally, you can add aquatic plants to your aquarium, which can compete with algae for nutrients and help to control their growth.

Too Much Light.

fish tank is too bright

Sunlight, LED lights and fluorescent light promote the growth of green algae. While all light sources enhance algae breeding, LED lights cause the most algae multiplication and are the most popular since they are affordable. Therefore, it’s best to avoid overexposing your fish tank to natural or artificial light for long periods. I have my lights on in my tank for no more than 10 hours a day. So far, I have managed to avoid any algae outbreaks.

Excess Nutrients.

Green algae grow rapidly in the presence of excess nutrients and minerals. These nutrients are in the following substances:

  • Leftover fish food
  • Dead fish 
  • Rocks and corals
  • Phosphates from tap water

Avoid overfeeding the fish, remove food leftovers immediately, and use rocks and corals in moderation to prevent the growth of green algae in the tank.

Not Changing the Water Regularly Enough.

It is critical to regularly change your fish tank water to help eliminate remnants of fish food. Also, phosphates and calcium build-up in water creates a rich breeding ground for algae. If your tap water is rich in phosphate or magnesium, consider filling the tank with distilled or reverse osmosis water instead.

Bacterial bloom in a newly set up tank.

A bacterial bloom in the first 6 weeks into a new tank is totally normal. It is known as New Tank Syndrome. This happens because it takes anywhere between 4-12 weeks for the water conditions in a new tank to stabilise. It is nothing to worry about and usually clears itself as the water starts to settle down.

Is Green Algae Dangerous?

Green algae is not dangerous to your fish unless it grows in excess. It is a living aquatic plant whose forms, shapes and sizes enhance the aesthetics of the aquarium. Also, during respiration, algae intake carbon dioxide and release oxygen that helps to maintain pH levels in the fish tank.

However, unrestricted growth of algae can affect your fish and the fish tank in several ways.

  • Respiration: Without sunlight or artificial light, respiration in algae is similar to that of animals. It uses oxygen and emits carbon dioxide. The result is a rise in the pH level in the water, making it acidic and uninhabitable for fish.
  • Decomposition: Dead algae decomposes inside the fish tank and uses up oxygen. In turn, the shortage or lack of oxygen causes the death of the fish in the tank.
  • Muddy-looking water: Excess algae makes the water look dirty and ruins the beauty of the fish tank. Cleaning the tank requires significant energy and time to maintain its beauty and the health of your fish.

How To Get Rid of Green Algae in a Fish Tank.

Green algae grows rapidly and can be challenging to remove. If left unattended, it can ruin the ecosystem in your fish tank. However, with proper care, you can maintain the health of your aquarium. 

Here is how you can get rid of green algae in your fish tank:

  1. Take action as soon as you notice it starting to form (this only refers to an established tank. If it is a new tank, you need to be patient).
  2. Clean the tank by carrying out a partial water change.
  3. Introduce algae eaters.
  4. Use a chemical algae remover.

The following sections will explain these steps and outline how to execute each one.

1. Take action as soon as you notice it starting to form.

Green algae can occur naturally about six weeks into a newly set up fish tank, as the tank establishes its nitrogen and bacteria cycle. It should clear on its own as the cycle stabilises. Also, plants in a new tank grow slower and absorb fewer nutrients than those in an older tank.

If you have an established tank, you need to do something about it, as the greener it gets, the harder it is to clear.

2. Clean the Tank.

The most effective way to remove green algae is to clean the tank manually. Use scrapers to scrape off the tank walls and sponges to wipe them off.

Clean the leaves and stems of your aquatic plants thoroughly. Also, remove and clean all your tank decorations.

3. Introduce Algae Eaters.

Algae eaters are an appealing, chemical-free way to remove green algae from your fish tank, especially if you have non-aggressive fish and a freshwater tank. Algae eating shrimp, snails, and fish all have unique habitat requirements. Therefore, you need to pick what is compatible with your tank or adjust your tank conditions accordingly.

I highly recommend you get a few nerite snails. I have 2 in my tank and they are my clean up team. My glass, heater, plants and ornaments are all a lot cleaner since they joined my fishy family.

nerite snail
One of my clean up guys.

It is advisable not to add algae eaters to your new fish tank too soon. Wait until the conditions in the tank are established, and the algae is more obvious.

4. Use a Chemical Algae Remover

If all other methods fail and your tank still has excess algae, you can treat it using chemicals to kill the algae and inhibit future growth. 

  1. Drain the tank completely and clean all the surfaces
  2. Refill the tank with clean water
  3. Add the chemical algae remover.

You can get the chemical pellets from your local fish shop, or from Amazon here. They are necessary because some algae may remain even after draining your fish tank.

How To Prevent Green Algae.

All fish tanks have some algae, and it’s often nearly impossible to eliminate. However, there are ways to control the growth of green algae in your fish tank.

  • Check the tank’s placement. Position your fish tank away from the windows to reduce direct sunlight. Minimise your lighting intensity if it is artificial, and only leave the lights on for 8 – 10 hours each day.
  • Watch out for excess food. Excess food, minerals, and nutrients stimulate the growth of green algae. Generally, if the fish do not finish the food within five minutes, the portion size is excessive and you need to reduce it.
  • Change the water. With time, phosphate and nitrate accumulate and promote algae growth. While you should not change all the water in the tank at once, changing up to 50% per week keeps nutrient levels balanced.
  • Test the water. Check the pH, mineral, and nutrient levels in your fish tank and make the necessary changes. A new tank requires frequent testing, while an old tank can have regular tests to maintain a healthy system.
  • Add live plants. Aquatic plants consume the same nutrients as algae, meaning fewer nutrients for the algae. About 25% of the fish tank’s ecosystem should be live plants to change the water quality.
  • Invest in a water filter. You have to take extreme measures to combat algae growth if it persists through water changes. Do not underestimate your tank’s filtration requirements. Get a filtration system that is approximately one and a half times or double the size of the fish tank.
  • Consider algae eaters. Some aquarium animals eat the algae and inhibit their multiplication. Before purchasing them, ensure their needs are compatible with your fish.
  • Use chemicals. Algaecides and other chemicals, such as copper sulphate, prevent algae breeding. However, it is best to use them in moderation because they can harm or kill your fish.


Green algae in a fish tank is inevitable in some form, from a small amount on your glass, to a full on explosion, but you can keep it under control with proper preparation. 

The best way to prevent excess green algae growth is to properly maintain your tank. You can do this by changing the tank’s water frequently, introducing plant life, and monitoring the amount of light the tank is exposed to each day.

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