Cloudy aquarium water: what each colour means and how to clear it
Milky white, green, or a brown tint — cloudy water is three different problems with three different fixes, and only one of them is dangerous to your fish.
Read the colour first
Cloudy water is not one problem. Hold a white card behind the tank, or look at a glass of tank water against a white wall, and decide which of these you are seeing — because the fix for one makes another worse.
Milky white, like watered-down milk. A bacterial bloom: free-floating bacteria multiplying faster than the filter can catch them, because something is feeding them. In a tank less than six weeks old this is the nitrogen cycle starting, and it is normal. In an established tank it means a sudden surplus of food for bacteria — overfeeding, a dead fish or snail somewhere, a filter that was cleaned too hard, or a big gravel disturbance that released trapped waste.
Green, like pea soup. An algae bloom in the water column itself, not on the glass. It needs two things to happen: light and nutrients. Too much light — a tank in a sunny window, or lights on twelve hours a day — plus nitrate or phosphate to grow on. Green water is ugly but harmless to fish in the short term; the risk is that a heavy bloom crashes overnight and takes the oxygen with it.
Brown or yellow tint, but clear. Tannins, from driftwood, leaves, or some substrates. It is not cloudiness at all — you can see through it — and it is harmless; many keepers deliberately create it. It lowers pH slightly, which some fish love, and fades with water changes over weeks.
Grey-white and fine, right after setup. Dust from substrate that was not rinsed enough. Settles in a day or two and the filter takes care of the rest.
What actually matters: the readings behind the cloud
Cloudy water itself does not harm fish. What harms them is what is often happening alongside it, and the only way to know is to test.
A bacterial bloom is feeding on something, and that something usually produces ammonia. In a new tank the ammonia is the cycle; in an established tank it is a problem. A green bloom means nitrate and phosphate are high enough to grow algae on, which also means water changes have fallen behind. The readings the app calls safe: ammonia 0, nitrite 0, nitrate under 20 ppm (40 is the ceiling), phosphate under 1 ppm.
If ammonia or nitrite is above zero, that is the emergency, and the cloud is a side note: 30–50 % water change today, test daily, and look for the source — count the fish, check behind the hardscape for a body, halve the feeding.
In the app — log the test and the verdict card tells you in one screen whether the cloud is cosmetic or a symptom: the score, the readings that are off, and the water change already calculated for your tank’s volume. A cycling tank gets its own read, because ammonia mid-cycle is progress, not a problem.
Clearing a bacterial bloom
The instinct is to change all the water and scrub the filter. Both make it worse: they remove the bacteria that were about to win, and leave the food for the ones that float. Instead:
- Stop feeding for two days, then feed once a day, only what is gone in a minute. This removes the surplus the bloom is living on.
- Leave the filter alone. The bloom clears when the filter’s bacteria outgrow the free-floating ones. A filter that was just cleaned is why the bloom started; cleaning it again resets the clock.
- Do modest water changes — 20–25 % every couple of days — for the ammonia, not the cloud. Big changes just dilute the bacteria that would clear it.
- Wait. A bacterial bloom clears itself in a few days to two weeks, often overnight once the balance tips. In a new tank it is part of the cycle and ends when the cycle does.
Water clarifiers clump the bacteria so the filter can catch them and give a clear tank in hours. They do nothing about the cause, so use one for a photograph, not as a treatment.
Clearing green water
Green water is a light problem first. Cut the light: lights on six to eight hours a day on a timer, and block direct sun on the tank with a background or a blind. That alone clears a mild bloom in a week or two.
For a stubborn one: a three-day blackout — lights off, a towel over the tank — starves the algae, and fish are fine without light for three days. Change 25 % of the water on each of those days to remove the dead algae, and feed lightly so the fish’s waste does not replace what the algae were eating. Fast- growing plants (hornwort, floating plants like frogbit) outcompete algae for the same nitrate and are the long-term answer. A UV steriliser on the filter outlet kills free-floating algae reliably and is the fix for tanks that get sun no matter what.
Then find the nitrate: if it is over 20 ppm, the water changes need to be bigger or more frequent, or the tank has more fish than the filter and the change schedule can carry.
In the app — Finley’s weekly check-in shows each tank’s score against last week and the tasks that got done, so a nitrate that climbs between tests shows up as a trend rather than a surprise. The schedule advisor suggests a tighter water-change cadence when the numbers say so, and you apply it with one tap.
Tannins and dust
Tannins fade with normal weekly water changes; activated carbon in the filter removes them faster if you want crystal water, and a piece of driftwood can be boiled for an hour to release most of its tannins before it goes in. Substrate dust settles by itself; a filter floss pad catches the last of it and can be thrown away after.
The one cloud that never clears on its own is the one that is being fed. Test, find the source, and the water follows.