Why is my new fish tank cloudy?
Three different cloudinesses with three different causes, and the instinctive response makes two of them worse.
- Difficulty
- intermediate
- Time
- 30 min
- Read
- 4 min
- Safety
- caution
Short answer
Grey or milky white cloudiness in a new tank is a bacterial bloom and clears itself in a few days — leave it alone. A fine haze on the day you fill it is disturbed substrate and settles. Green water is algae driven by light and nutrients. Test ammonia and nitrite first, because that decides whether anything is actually wrong.
The reflex when a tank goes cloudy is to change all the water and clean the filter, and both are precisely wrong. Cloudiness in a young tank is usually a sign that the biological system is working through its startup, and interrupting it prolongs exactly the phase you want to end. Distinguishing the three types takes thirty seconds and decides the response.
Safety
Step by step
- Test before doing anything.Ammonia, nitrite, nitrate and pH with a liquid kit. Cloudiness with zero ammonia and zero nitrite is cosmetic. Cloudiness with either present is a water quality problem and needs action.
- Identify which cloudiness it is.Milky grey is bacterial. A settling haze on day one is substrate dust. Green is algae. Brown usually means tannins leaching from new wood, which is harmless and fades.
- Resist the urge to change all the water.A full water change removes the dissolved organics but not the cause, strips the tank's stability, and stresses fish through sudden chemistry and temperature changes.
- Never clean the filter thoroughly during a bloom.The bacterial colony lives in the media. Rinsing it under the tap kills it and restarts the cycle with fish present, which turns a cosmetic problem into a lethal one.
- Cut the feeding right back.Uneaten food is the fuel. Feed what is cleared in a couple of minutes, once a day, and skip a day entirely during a bloom.
- Check the stocking level honestly.A tank that clouds repeatedly is often simply overstocked for its filtration. That is a permanent problem that no maintenance routine fixes.
- Put the lights on a timer.A fixed daily period rather than on whenever someone is in the room. Excess light is the main driver of green water, and a timer removes the variability.
- Keep a log of test results.A dated note of ammonia, nitrite and nitrate. The pattern over a fortnight tells you far more than any single reading, and it is what an aquatics shop will ask for.
If it doesn't work
Milky, grey or white cloudiness in a tank set up in the last few weeks
Cause: A bacterial bloom — free-floating heterotrophic bacteria multiplying on dissolved organics while the filter colony establishes — Fix: Leave it. Do not change all the water and do not clean the filter. Feed lightly or not at all if there are fish, test ammonia and nitrite daily, and do a partial water change only if either is detectable. It clears in a few days to a fortnight.
Fine haze appearing the moment the tank was filled
Cause: Substrate dust suspended in the water — Fix: Wait. It settles or is removed by the filter within a day. Rinsing substrate more thoroughly before filling avoids it next time.
Green water
Cause: Free-floating algae, driven by too much light and available nutrients — Fix: Reduce the lighting period to a set number of hours on a timer, move the tank out of direct sunlight, feed less, and do regular partial water changes. Live plants compete with algae for nutrients and help substantially.
Cloudiness returning weeks after the tank looked established
Cause: Overfeeding, overstocking, a dead fish or plant decaying unnoticed, or a filter that has been over-cleaned — Fix: Count the fish, look behind the decor and under the substrate, reduce feeding, and test the water. Restore the filter's biological media and stop rinsing it under the tap.
Cloudy water with fish gasping at the surface or hanging near the filter outlet
Cause: Poor water quality, most likely ammonia or nitrite — Fix: Test immediately. Where either is detectable, do a partial water change of around a quarter with dechlorinated, temperature-matched water, repeat daily, and stop feeding until the readings fall. Do not add more fish.
Questions people ask
How long does a bacterial bloom last?
Usually a few days to a fortnight, and it clears on its own. Water changes and filter cleaning prolong it rather than shortening it.
Should I use a water clarifier?
It clumps particles so the filter can remove them, which treats the appearance rather than the cause. It is not harmful in most cases but it does not address overfeeding, overstocking or an immature filter.
My water is green, not cloudy — is that different?
Yes. Green water is free-floating algae driven by light and nutrients. Reduce the lighting period, move the tank out of direct sun, feed less and change water regularly. Live plants help by competing for the same nutrients.