The short answer
Milky, white or greyish cloudiness is almost always a bacterial bloom — a sudden surge of free-floating bacteria feeding on dissolved nutrients in the water. It’s harmless and extremely common in new tanks, and it typically clears on its own within a few days once the bacteria run out of food. It’s one of the few aquarium problems where the best action is patience.
Why it happens
A bacterial bloom appears when there’s a spike of nutrients — dissolved organics from fish waste, uneaten food, or a maturing substrate — for bacteria to feast on. They multiply so fast that the colony clouds the water. This is different from green water (algae) or a tint from tannins; a bloom is a distinctly white, milky haze that light beams through.
New tanks get it most because the biological filter is still establishing and there’s plenty of loose nutrient around. A deep filter clean or big substrate disturbance in an established tank can trigger a smaller one too.
What to do — and not do
- Wait it out. The bloom fades once the bacteria exhaust their food source, usually within a few days.
- Don’t overfeed. Extra food is extra fuel for the bloom, so ease off feeding while it clears.
- Don’t over-clean the filter. Scrubbing everything just resets the cycle and can prolong things.
- Keep testing in a new tank — the cloudiness is harmless, but you still want to watch ammonia and nitrite.
Next steps
Because this is so often a new-tank thing, see is it normal for a new tank to be cloudy? and what is a bacterial bloom? for the full picture. If your haze is green or brown instead of white, see why is my aquarium water cloudy?. Keep an eye on parameters with a water test kit, and browse water testing for more.