White / milky water — a bacterial bloom
By far the most common, especially in new tanks. A sudden explosion of free-floating bacteria clouds the water grey-white. It's usually triggered by a new tank still cycling, overfeeding, or disturbing the substrate.
Fix: don't panic and don't do giant water changes — that just feeds the cycle. In a new tank, let it finish cycling; the bloom clears on its own in a few days to two weeks. Feed lightly, don't add more fish, and make sure your filter media isn't being over-cleaned. If it's an established tank, it usually means something decomposing (uneaten food, a dead fish, a buried snail) — find and remove it.
Green water — an algae bloom
A pea-soup green tint is free-floating algae, fueled by too much light and too many nutrients (nitrate and phosphate). Direct sunlight on the tank is a frequent culprit.
Fix: cut the lighting to 6–8 hours a day, move the tank out of direct sun, stop overfeeding, and bring nutrients down with water changes. For a stubborn bloom, a UV sterilizer clears it fast. Getting your nitrogen cycle under control prevents it coming back.
Brown / yellow water — tannins
New driftwood leaches tannins that stain the water like weak tea. It's harmless — actually beneficial for many soft-water fish — but not everyone wants the look.
Fix: it fades over weeks. Speed it up by pre-soaking or boiling new wood, running activated carbon, and doing regular water changes.
When cloudy water is actually dangerous
Cloudiness itself rarely hurts fish — but the cause can. If fish are gasping at the surface, clamped, or lethargic alongside the cloud, stop guessing and test the water for ammonia and nitrite. In a new tank, a bloom plus stressed fish usually means the tank isn't cycled and ammonia is rising.
Know which cloud you've got
TankMate lets you snap a photo of the tank for an AI read on what's happening, log your water tests to catch the real trigger, and track your Tank Health Score so you see problems before the water turns. Free on iOS and Android.