Aquarium Maintenance

Why is my fish tank cloudy?

Why is my fish tank cloudy?

Why is my fish tank cloudy? This is a common question in fish keeping. Cloudy tank water is a common problem in freshwater. The water may be gray or off white, yellow or green. Each color indicates a different ongoing problem with the aquarium, whether it is tannins in water, free floating algae or a bacteria bloom.

In many cases, cloud water tanks do not pose a significant threat to your fish but may affect the appearance of your tank and may indicate other problems such as low water quality.

That is why in this article we will look at the causes, solutions and ways to prevent exposure to cloud water.

See also the complete care guide for Platy fish

Possible causes of cloud tank.
Bacteria bloom

The most common cause of cloudy water in a freshwater aquarium is bacterial bloom. Bacterial bloom occurs when there is a sudden increase in the number of bacteria present in the tank – high concentration of bacteria in tank water often leads to a cloudy or milky appearance of the water.

Bacteria Bloom in new Aquarium

Bacteria bloom are mostly common in new fish tanks when nitrogen cycle has not taken place. The nitrogen cycle is a process in which beneficial bacteria in your tank work to break down waste, converting toxic substances such as ammonia and nitrite into less harmful nitrates.

These bacteria thrive in tanks where there is a large amount of organic waste and in tanks where ammonia and nitrite levels are very high. If your tank has not been properly cycled, or if it has a large amount of natural debris built up in the substrate, you may experience a bacterial overgrowth.

Bacteria Bloom Due to Sudden Increase In Nutrition Levels

Introducing a lot of fish into your aquarium tank or heavy feeding, even after cycling your fish tank, will increase ammonia and nitrites levels causing it to rise and this can be harmful to your fish. Beneficial bacteria multiply rapidly to cope with the additional waste produced, which in turn causes the cloudiness in fish tanks.

How to Deal With Bacterial Bloom

Bacterial bloom often occurs when the tank does not produce enough beneficial bacteria to process fish waste. Regardless of the cause, do not panic. Here’s how to deal with it for good;

No Fish in the Tank

If there are no fish in the tank, such as in a fish-free cycle, there is no need to do anything about bacteria bloom. Allow about three weeks for the tank cycle process to be completed before putting in the fish. Just let it continue its course and it will clear once the heterotrophs has eaten up the organics and die off.

Fish in the Tank

  • Allow about 1-2 days for the floating bacteria to settle and the water will clear.
  • Reduce feeding to every second or third day, which will uneaten food in the tank.
  • Make 50% water changes daily to help remove organic compounds from water and substrate.
  • Keeping the aquarium very clean by removing debris such as rotting plants, uneaten food, and clean the gravel regularly.
  • Clean your filters using Tank water. About once a month, it is a good idea to clean your filters.
  • Do not sterilize, that can kill the useful germs you need, but you can clean the mud that builds up over time.
Algae bloom

Another possible cause of cloudy tank water is the algae bloom. Algae blooms causes the tank water to be green. Algae bloom may occur when the tank is exposed to high levels of light or carbon dioxide and when there is excess nutrients in the tank. If your aquarium is in direct sunlight or if you leave your tank lights on for more too long, this may result in algae growth-algae bloom. Also excess nutrients from waste build up can also lead to an algae bloom.

How to remove algae bloom

  • Use RO water or phosphate remover to treat the water.
  • Remove the algae using algae scrappers and move the aquarium tank away from direct sunlight, or Reduce amount of time the lights are on.
  • Provide quality food in the right amount to reduce the amount of uneaten food.
  • Remove any uneaten food in the tank.
  • Avoid overfeeding to reduce the amount of fish waste
  • Clean your filters using tank water. About once a month, it is a good idea to clean your filters.
  • Avoid putting too many fish all at once in the tank as this may cause waste build up overtime.
Dirty substrate

A third factor in the cause of cloud waterlogging is the contaminated substrate. If you put gravel, sand or new soil in your tank, a cloud of dust or detritus can end up in a column of water.

The substrate bags are rotated, tossed to and fro and routed as they go from manufacturer to store. All such movement means that the substrate inside the bag rubs itself and breaks down the tiny particles that make up fine dust.

If you don’t wash the substrate enough, it mixes with a large cloud and forms a lot of dirt.

How to fix cloudy tank from dirty substrate

When setting up your tank for the first time you need to rinse off dirt from your gravel substrate before putting it in the tank. When preparing your substrate for use, place it in a large plastic container or bucket and transfer cool water over it.

Use your hand to shake the substrate as the water runs, allowing it to drain excess to the sides of the container. When the water in the container starts to clear (meaning there is no dust or dirt left), your substrate is clean and ready for use.


Sometimes, aquarium water can turn yellow / brown. This is due to the tannins in the water. Don’t panic about it. Tannins will not harm your fish or plants. You see, wood, leaves and seed pods emit brown dye when soaked in water. This brown dye is called tannin.

In the aquarium, the most common source of tannins is drift wood, as tannins slowly enter your aquarium, it will make your water turn a dark yellow color. Plants produced this as a defense mechanism. They give plants a strong bitter taste that can help keep insects from eating them.

How to get rid of it!

Most tannins can be removed by soaking the wood before adding them to your aquarium. Boiling speeds up the process.

When it’s too late, and you’ve already put the wood in your aquarium, remove it and soak it. Any tannins you remove now will not dye your aquarium water when you put the driftwood back in your tank.

But what if the water inside your aquarium has already dyed brown or yellow?

Your best bet is to use activated carbon or another type of chemical filter, such as using chemical media which will remove tannins from the water leaving you with crystal clear water.

Preventing cloudy tank water

Tannins, bacteria bloom and microalgae can be a real pain, but there are ways to eliminate them permanently. More often than not, things like gray and green water are caused by a certain imbalance in the tank.

Adequate maintenance routine can be great help in coping with these problems and keeping them off long term.

To prevent recurrence of cloud water tanks you must take a few critical steps seriously;

  • Avoid over-feeding your fish as any uneaten fish food will simply sink to the bottom of the tank and be destroyed as garbage, providing bacteria and algae with vital nutrients.
  • You should also make sure you keep up with your regular water changes every week – change between 10% and 20% of the volume of your tank each week and make sure to siphon some aquarium substrate each time the water changes.
  • Changing your filter media every month will help ensure that your filter is working properly.
  • Removing dissolved wastes and solid debris from your tank water.
  • Clean the tank regularly

When you take these steps, you should have no problem keeping the water quality of your tank high. If you clean your tank, you are less likely to have a problem with tank water.