Guide

What to do if aquarium water is cloudy

By the Aquarium Calculator team Updated 9 October 2026

First, look at the colour. Milky white water in a new tank is usually a harmless bacterial bloom that clears on its own in one to two weeks. Grey haze is loose particles, green is free-floating algae, and yellow-brown is tannins from wood. Test ammonia and nitrite: if either is above zero, do a 30–50% water change right away.

Identify the cloudiness by its colour

Cloudy water is a symptom, not a single problem, and the colour is the best clue to the cause. Milky white or hazy water, especially in a tank that is only a few weeks old, is almost always a bacterial bloom. Grey or beige cloudiness that appears right after setup or after moving decorations is fine particles from the substrate. Green, pea-soup water is free-floating algae. Clear but yellow or tea-coloured water comes from tannins released by driftwood, leaves or some substrates.

Two quick checks narrow it down further. If the haze settles when the filter is off for a few minutes, it is particles; if it stays evenly suspended, it is bacteria or algae. Then test the water: ammonia, nitrite and nitrate tell you whether the cloudiness is linked to a water quality problem that needs immediate action, or just a cosmetic one that will pass.

Milky white water: bacterial bloom

A bacterial bloom happens when free-floating bacteria multiply quickly on a surplus of nutrients, typically in a new tank that has not finished cycling, after overfeeding, or after a filter has been cleaned too hard. The bacteria themselves are not harmful, and the bloom usually clears within one to two weeks once the beneficial bacteria in the filter and substrate catch up and use the excess nutrients.

The best fix is patience and restraint. Do not replace the filter media or scrub everything clean, because that removes the bacteria that will end the bloom. Feed lightly, every other day if the tank is new, and keep testing. If ammonia or nitrite rises, do partial water changes of 25–30% to keep them low. Avoid adding more fish until the water has cleared and the tank tests at zero ammonia and zero nitrite.

Green water: free-floating algae

Green water is caused by microscopic algae suspended in the water. It thrives on too much light, especially direct sunlight or lights left on for 10 hours or more, combined with excess nutrients from overfeeding, overstocking or infrequent water changes. Unlike algae on the glass, it cannot be wiped off, and water changes alone often only dilute it briefly.

Cut the light to 6–8 hours a day and move the tank out of direct sunlight. A complete blackout for three to four days, with the tank covered and lights off, starves the algae; fish and most plants tolerate it. Reduce feeding, keep up weekly water changes, and consider an inline UV steriliser, which clears green water reliably within a few days. Fast-growing plants compete with algae for nutrients and help stop it returning.

Grey haze and yellow water: particles and tannins

Grey or beige cloudiness in a freshly set-up tank is dust from sand or gravel that was not rinsed well enough. It usually settles within a day or two. A pad of fine filter floss in the filter speeds this up; replace it once it is dirty. For the future, rinse sand and gravel in a bucket until the water runs almost clear, and fill a new tank slowly over a plate or bag so the stream does not stir up the bed.

Yellow or brown water from driftwood, catappa leaves or peat is caused by tannins. It is harmless and many fish from soft-water rivers enjoy it, but if you prefer clear water, boil or soak new wood for a few days before use, run activated carbon in the filter, and keep up regular water changes. The colour fades as the wood ages.

When cloudy water is dangerous, and how to prevent it

Cloudiness becomes urgent when it comes with fish gasping at the surface, staying near the filter outlet, or acting lethargic, or when the water smells bad. These signs point to an ammonia or nitrite spike, low oxygen, or something decaying such as uneaten food or a dead fish hidden behind decorations. Test immediately, remove anything rotting, and change 30–50% of the water with dechlorinated water at the same temperature. Repeat daily until ammonia and nitrite are back to zero.

Prevention is simple. Feed only what the fish eat in a minute or two, keep the stocking level comfortable, size the filter correctly for your real water volume, and change 20–30% of the water every week. When you clean the filter, rinse the media in old tank water rather than tap water, and never replace all of it at once. The water change calculator works out exactly how much to replace for your tank.

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