Cloudy water is usually one of two things. If it followed new sand or stirring the sand bed, it’s fine particles: change your filter socks or floss daily until it clears. If the tank went hazy overnight with no obvious cause, it’s probably a bacterial bloom: keep surface movement and the skimmer running, remove the source, and wait. Blooms usually clear within one to two weeks.
Fine particles
Bulk Reef Supply says this follows new sand or disturbing the sand bed. Replace mechanical filtration every 12–24 hours until it clears. A clarifier can help with live sand, and a separate polishing filter clears severe cases faster.
Rinse dry sand well before adding it next time.
A bacterial bloom
A bloom appears suddenly: clear yesterday, hazy this morning. Bulk Reef Supply links it to a sudden rise in organic matter, such as a new tank cycling, a dead fish, overfeeding, too much liquid food or a disturbed dirty sand bed.
- Remove the source: anything dead, uneaten food, excess liquid feed
- Keep the skimmer running and point a wavemaker at the surface for oxygen
- Clean only a small patch of sand at each water change
- Wait: Bulk Reef Supply says blooms usually clear in 1–2 weeks
Watch the oxygen
Bulk Reef Supply warns that bacteria in a bloom can use up oxygen faster than the fish. If fish breathe fast or hang at the surface, increase surface movement or add an airstone straight away.
It also warns that a water change may seem to help, but the bloom often comes back worse.
Cloudy after a water change
A white haze straight after a water change can come from the new salt. Red Sea links it to overheating, over-mixing or too much salt, and says it’s harmless. See how to do a water change.
Common questions
Is cloudy water dangerous for fish?
It can be, mainly through low oxygen in a bloom. Keep the surface moving.
Should I do a water change for cloudy water?
For particles it can help. For a bacterial bloom, Bulk Reef Supply says it often comes back.
How long does a bacterial bloom last?
Usually one to two weeks.
Sources and limits
Last checked 13 October 2026. This guide brings together the published advice below. It can’t see your tank: if livestock is in distress, act first, then ask an experienced local shop or club, ideally with your test log to hand.