The short answer
A bacterial bloom typically lasts anywhere from a couple of days to two weeks, most often clearing within 3–7 days. It ends when the free-floating bacteria run out of the dissolved nutrients feeding them — at which point the population crashes and the water goes clear, often quite suddenly. Patience is the cure; over-changing the water just feeds it and drags it out.
What affects the timing
How long a bloom lasts depends on how much food is available and how quickly it’s used up:
- New tanks bloom longest — there are lots of nutrients and no established balance. Expect up to a week or two.
- A one-off disturbance (adding wood, stirring substrate) usually clears in a few days.
- Ongoing overfeeding keeps supplying food and can make a bloom seem endless.
- Repeated large water changes add fresh nutrients and, counter-intuitively, prolong it.
Helping it along
You can nudge a bloom to clear faster by removing its food, not by cleaning harder:
- Feed lightly or skip a day to cut the nutrient supply.
- Add fast-growing live plants to compete for the dissolved nutrients.
- Keep good flow and filtration so surfaces can grow the beneficial bacteria that outcompete the bloom.
- Wait it out — a healthy new tank always clears eventually.
When to worry
The haze is harmless, but in a cycling tank the ammonia and nitrite behind it are not. Keep testing with a liquid test kit and act on those readings, not the cloudiness. For the full picture, see what a bacterial bloom is and how to cycle an aquarium. A bacteria starter helps establish the surface bacteria that end blooms for good.