FAQ deep-dive
How to Get Crystal-Clear Aquarium Water
Why your aquarium water is cloudy, white, green or brown, and how to fix each cause, from bacterial blooms and green water to fine debris and tannins.
Updated 3 min read
Part of our complete planted-aquarium guide.
The short answer
Cloudy water is not one problem, it is several, and the fix depends on the colour. White or milky is a bacterial bloom, normal in a new tank, and it clears itself if you leave it alone and feed lightly. Green is a free-floating algae bloom from too much light and nutrients, fixed with a blackout, a UV steriliser, or cutting light. Yellow or brown is tannins from driftwood, harmless and easy to reduce with carbon and water changes. A fine grey dust is disturbed substrate or debris, which the filter catches within hours. Work out which one you have, and the fix follows.
White or milky water: a bacterial bloom
A white haze that makes the tank look like watered-down milk is a bacterial bloom, and it is the most common cloudiness in a new tank. When there is a surplus of nutrients and the surfaces are not yet fully colonised, bacteria multiply free-floating in the water column until they run out of food and settle down. It looks alarming and is completely harmless.
The fix is patience. It clears on its own in a few days to a couple of weeks. Crucially, do not do big water changes, which just reset the balance and prolong it. Feed the fish lightly, make sure the tank is properly cycling, and wait. Heavy planting helps, because plants take up the nutrients the bacteria feed on.
Hornwort
Ceratophyllum demersum
Cosmopolitan
Green water: an algae bloom
Green, pea-soup water is a bloom of single-celled algae floating in the water column. It is driven by too much light and nutrients, and direct sunlight on the tank is a classic trigger. Ordinary algae grazers cannot touch it because it floats, so water changes barely help.
Three fixes work:
- Blackout: cover the tank completely for three days, no light at all. The algae starves and dies. Feed sparingly and it does not harm the fish.
- UV steriliser: clears green water within days by killing the free-floating cells as water passes through.
- Fix the cause: shorten the photoperiod, move the tank out of direct sun, and don't over-fertilise.
See the green water page in the algae hub for the full method.
Yellow or brown water: tannins
Tea-coloured water is almost always tannins leaching from driftwood. It is harmless, mildly softens and acidifies the water, and many keepers deliberately create this blackwater look. To reduce it: pre-soak or boil new wood before it goes in, run activated carbon in the filter, and do water changes. It fades over weeks as the wood exhausts its tannins.
Grey dust: debris and disturbed substrate
A fine grey or brown haze right after planting, rescaping or a substrate stir is just suspended particles. The filter catches it within hours. A polishing filter pad or fine floss speeds it up. If it keeps happening, your flow may have a dead spot where detritus settles and re-suspends, or you are overfeeding.
Keep it clear for good
Clear water is mostly a by-product of a balanced, well-kept tank:
- Feed lightly. Excess food is the fuel behind most cloudiness.
- Keep up water changes and filter maintenance. See the maintenance routine.
- Plant heavily. Plants out-compete the free-floating algae and bacteria.
- Run a clean-up crew. Shrimp, snails and algae grazers keep surfaces clean.
Amano Shrimp
Caridina multidentata
Japan, Taiwan
Zebra Nerite Snail
Vittina natalensis
East Africa (coastal rivers)
FAQ
The quick answers are above. If your problem is specifically green or growing on surfaces, head to the algae hub. For a new tank that has never been clear, start with cycling.
Frequently asked questions
Direct answers to the questions search engines and AI assistants surface most often about this species.
Why is my aquarium water cloudy and white?
A white or milky haze is almost always a bacterial bloom, most common in a new tank. Free-floating bacteria multiply on excess nutrients before the surfaces colonise. It clears on its own in a few days to a couple of weeks. Do not do big water changes, which just feed it; feed the fish lightly and wait.
Why is my aquarium water green?
Green water is a free-floating algae bloom, caused by too much light and nutrients, often direct sunlight or an over-long photoperiod. Fix it with a three-day blackout (cover the tank completely), a UV steriliser, or by cutting light and nutrients. Water changes alone rarely clear it.
Why is my aquarium water yellow or brown?
Usually tannins leaching from driftwood, which stain the water tea-coloured. It is harmless and many keepers like the blackwater look. To clear it, pre-soak or boil new wood before adding it, use activated carbon, and do water changes. It fades over weeks as the wood exhausts its tannins.
How long does cloudy water take to clear?
A new-tank bacterial bloom usually clears in a few days to two weeks on its own as the bacteria settle onto surfaces. A dust cloud from disturbed substrate settles in hours once the filter catches it. Green water needs active treatment and can take a week or so to fully clear.
Will a water change clear cloudy water?
It depends on the cause. It helps with fine debris. It does little for a bacterial bloom (and can prolong it) or green water. Diagnosing which kind of cloudiness you have matters more than reaching for water changes.