The short answer
A bacterial bloom is a cloud of free-floating (heterotrophic) bacteria multiplying rapidly in the water column, turning it milky white or grey. It’s extremely common in new tanks and after a big disturbance, as bacteria feed on a sudden supply of dissolved nutrients. It looks alarming but is harmless, and it clears on its own within days once the food source runs out.
Why it happens
The bloom is different from the bacteria that make up your biological filter. Your filter bacteria (nitrifiers) live on surfaces — media, gravel, glass. A bloom is a different, faster-growing group that floats freely and reproduces in hours when there’s plenty of dissolved organic food.
Common triggers:
- A brand-new tank with lots of available nutrients and no established balance yet.
- Adding a new substrate, driftwood or lots of food releasing a burst of organics.
- Over-cleaning that disturbs settled detritus.
What to do (mostly wait)
The bloom feeds on dissolved nutrients. Once it exhausts them, the population crashes and the water clears — usually in 2–7 days.
- Be patient and avoid the urge to over-change the water, which just adds nutrients and prolongs it.
- Don’t overfeed — feed lightly or skip a day.
- Keep testing ammonia and nitrite if the tank is cycling — those are the real concern, not the haze. Use a liquid test kit.
The takeaway
A bacterial bloom is a normal part of a tank finding its balance, not a sign something is wrong. Let it run its course. For how long to expect it, see how long a bacterial bloom lasts. If your tank is new, read how to cycle an aquarium and browse the filter hub.