White or gray haze in an otherwise normal-looking tank is almost always a bacterial bloom — a sudden spike in the bacteria that feed on excess waste or uneaten food. It's common in new tanks that are still cycling and in established tanks after overfeeding, and it typically clears on its own within 3-7 days.
How long does cloudy tank water take to clear?
Most bacterial blooms resolve within 3-7 days once the excess waste feeding them is reduced. If it's been longer than a week with no improvement, or the water has a green or brown tint rather than white/gray haze, you're likely looking at a different issue — see the comparison table below.
Should I do a water change for cloudy water?
A small water change is fine, but resist doing a large one. It's tempting to want to "flush out" the cloudiness, but a big water change removes a lot of the very bacteria that are working to stabilize your tank, and can end up prolonging the bloom rather than fixing it. A modest 10-15% change alongside reduced feeding is usually the better move.
What actually causes a bacterial bloom?
- A recent increase in waste — overfeeding, a new batch of fish, or uneaten food breaking down.
- A tank that's still cycling, where the bacteria population is naturally in flux as it establishes itself.
- Overcleaning the filter — rinsing media in tap water kills off beneficial bacteria, which can trigger a bloom as the ecosystem rebalances.
What to do about it
- Cut back feeding for a few days — less waste means less fuel for the bloom.
- Test ammonia and nitrite to rule out a bigger underlying issue rather than just a routine bloom.
- Do a modest partial water change if you want to speed things along, but avoid a large one.
- Otherwise, let it run its course. It's more of a cosmetic nuisance than a danger to your fish in most cases.
Cloudy, green, or brown — which is it?
| Water color | Likely cause | Guide |
|---|---|---|
| White or gray haze | Bacterial bloom | This guide |
| Green, pea-soup colored | Free-floating algae bloom | Green water guide |
| Brown or tea-colored | Tannins from driftwood or leaves | Brown water guide |
Want a definitive answer for your specific tank? Run the 2-minute water diagnosis — it separates a bacterial bloom from an ammonia spike or an algae issue based on exactly what you're seeing, and links to the full cloudy water guide.