Why is my fish tank water cloudy?

Cloudy water affects so many aquariums. Here are some of the things that cause it.

New gravel

The most common cause of cloudy water in new fish tanks is the gravel. When aquarium gravel and sand is brand new it will contain some dust. All new gravel and sand should be washed vigorously in a bucket, with tap water, until the water runs clear. If gravel isn’t prewashed before being added to the tank, when you fill it with water, the water will go cloudy.


Always give aquarium and sand an extra long rinse when bought new. You’ll rarely get it completely free of dust, but you can deflect water through a kitchen colander or onto a dish when you fill the tank, disturbance and cloudiness with be lessened. Plug the filter in after filling and water should clear of dust within 24 hours. Fine filtering materials like filter floss will aid mechanical filtration and speed up the clearing of the water. If it still won’t clear you can use a flocculant like Dr Tim’s Clear Up. Flocculants cause tiny particles to clump together into larger lumps, sinking to the bottom or being removed more easily by the filter. 

Bacterial bloom

If it’s not the gravel, the cloudiness may not be from tiny particulate matter. It may be bacterial. Bacterial blooms are common in new aquariums less than six weeks old, when the biological balance is not fully established. Millions of tiny bacteria may be present, but it may be loose in the water column, instead of in biofilms inside the biological filter. The result is milky water which won’t clear, with no apparent cause.


Ensure that your filter is large enough for the aquarium its running, and that it has a biological media chamber. Test water to ensure that no ammonia or nitrite is present (indicative of an immature aquarium,) add a flocculant, and some filter starter. If ammonia and nitrite is present, stop feeding, add an ammonia detoxifier, some filter bacteria, and change some water to relieve the stress on the fish.

Uneaten food

Most fish foods shouldn’t cloud water but some oily, protein rich foods can. Uneaten foods often cloud water too, and uneaten food is often a sign of overfeeding, or that the fish are sick from disease or bad water quality, and are off their food. 


If the food is clouding the water,  try a suitable food from another manufacturer. If the food is usually ok but is clouding when left uneaten, remove it with a net or a gravel vacuum. Test water to ensure that its ok, and observe your fish for any signs of disease. If water parameters are adverse, perform a water change and add some beneficial bacteria

Dead fish

A dead fish, snail, shrimp or rotting plant can cause cloudiness in the water. Dead fish corpses break down very quickly in tropical fish tanks and if they are sizeable, or a few have died at the same time, the result can be polluted water. 


Lift up ornaments and decoration to search for any dead fish, and if you find any, remove with a net. Test the water and if ammonia or nitrite is present, do a 50% water change, and add some filter bacteria. 

Fuzzy wood

There are many interesting types of wood now available, although some can develop a fungal growth when first submerged, and cloud water.


Pre Soak the wood in a separate tank or bucket until the wood sinks, and there is no further sign of fungal growth. Wood can be boiled to kill off any live fungus before being placed into the tank, and activated carbon will help to remove any staining and cloudiness. After a few months all fungal growths should stop, due to the last remaining nutrients in the wood, soaking out into the water.  

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Jeremy Gay is an author and freelance aquatic specialist. A former editor of Practical Fishkeeping magazine, he offers a wealth of experience on all things aquarium and pond.

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