Top Causes of Cloudy Water in Freshwater Tanks and How to Clear it Up

At some point in keeping our aquariums we may wake up or come home from work to find our freshwater tank is cloudy. Luckily, most of the time it is not an emergency, but naturally draws the question of what caused this to happen? The truth is, there are no clear answers. Cloudy water can be caused by different factors, but here are a few of the top causes you’ll find listed in the following, in no particular order:

Dirty Gravel

Cloudy freshwater is most common in newly set-up tanks. If the gravel wasn’t cleaned properly before being put in, the water when added can dislodge dust and cloud it up.

Rinse all new sand and gravel until the water runs clear, or add some filter floss or a pad to the filtration system to try and trap the tiny dust particles. The water should then clear over time.

Inadequate or No Filtration

Filtration is recommended for all sizes and types of aquaria. A lack thereof is one of the top causes of cloudy water in freshwater tanks. If you don’t run a filter on your tank, it can go cloudy from the fish poop and deadly ammonia build up that can harm fish. Install a filter on the tank and make sure it runs for 24/7. Be sure to maintain the mechanical, and chemical media regularly. In addition, dosing some beneficial bacteria to kick start the biological filter and also stock the tank slowly when starting out.

If you already have a mature filter and the water is cloudy, the filter system being ran may not be enough to cope with the amount of fish, food, or volume of the tank that its on. To test if it is inadequate, install another filter (if you have a backup laying around), at the other end of the tank and see if the water clarity improves over the coming days. Another option is to upgrade to a much larger filter with improved mechanical filtration to trap physical waste, and improved biological filtration to convert fish waste into less harmful substances.

Uneaten Food

Another one of the top causes of cloudy water stems from overfeeding or feeding a low quality fish food. Uneaten fish flakes and pellets should be removed with a net or siphon tube. If there’s food left on the bottom of the tank after you’ve fed the fish, you’ve fed too much and should adjust to feed less.

Bacterial Bloom

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Sometimes water tests can be fine and the fish healthy, but the water can go milky white for no apparent reason. The cause could be that bacteria may have multiplied and is floating around in the water column instead of sticking to the filter media and gravel, and “bloom.” Do nothing and it should go away, or add a treatment aimed at clearing bacterial blooms.

Stirred Gravel

If you have added or stirred up the gravel, you can speed up the clearing of the water by adding a flocculant, like Brightwell Clarifi-FW. A flocculant is an additive that binds tiny particles floating in the water column together, forming larger ones making them easier to capture using mechanical filtration. Although the clarity may get worse before it gets better, most flocculants work well resulting in crystal clear water. However, keep in mind that they should be used in conjunction with mechanical filtration.

Deceased Livestock

A dead fish or dead snail will quickly decay if not found and removed, causing the water to go cloudy. Do a quick count, and if something is missing lift some decor to try and find a corpse. If anything has died, check life-critical water parameters including temperature, oxygen, and ammonia, and rectify if needed. Check the other fish for visible signs of disease and use medication if necessary. Don’t add any more livestock until the water quality is better and the other fish in the tank are healthy.


Bogwood can make freshwater cloudy by releasing tannins into the water. Usually, they turn it the color of tea but sometimes wood can develop fungal growths which make the water go cloudy. Use activated carbon in the filter to remove the tannins and take the wood out and soak it until all the fungal growth has disappeared. 

Green Water

Unicellular algae can make a freshwater tank go green and cloudy. This can be because of too much light or excess nutrients in the water causing the algae to bloom. Reduce the tank lighting, use a flocculant or green water aquarium treatment, or fit a UV sterilizer long term.

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