Cloudy aquarium water: white, green or brown, what it means and how to clear it

Cloudy aquarium water is usually a harmless bacterial bloom in a new tank, and the best thing you can do is test the water and then be patient rather than scrubbing everything. The colour tells you the cause: white or grey usually means bacteria or dust, green means algae, and yellow-brown means tannins from wood, which are harmless.

The one cloudy tank you must not wait out is the one that smells bad, follows a dead fish or a big overfeed, or has fish gasping at the surface. That's rotting waste, an ammonia spike and falling oxygen, and it needs action today.

How to identify it

Scoop some tank water into a clear glass or white cup and look at it in good light. If the water in the cup is clear but the tank looks hazy, the haze may be on the glass, not in the water (Aquarium Co-op).

Milky white or grey, no visible specks: a bacterial bloom, millions of free-floating bacteria feeding on waste. Very common in the first weeks of a new tank while it cycles, after adding many fish at once, or after overfeeding.
Cloudy with visible specks that settle: particles: dust from new gravel or sand, stirred-up debris after cleaning, or crumbs of messy food.
Green (pea soup): an algae bloom of tiny floating algae. Usually too much light and spare nutrients. See algae takeover.
Yellow, brown or tea-coloured but clear: tannins, natural compounds that leach out of driftwood, leaves and other botanicals. Harmless, and many soft-water fish come from rivers coloured like this. Tannins can lower pH slightly.
Cloudy and smelly (rotten, sour or sulphur smell), often grey or yellowish: a rot bloom from a dead fish, a large amount of uneaten food, or a crashed filter.

Easily confused with: scratches or film on the glass; tiny bubbles from new water or a stirred substrate (they rise and disappear); and medicine colouring the water.

When it's a problem

Normal, don't worry: a white haze in a tank under two months old with healthy fish; dust for a day after adding gravel; tea-coloured water from new wood; a slight green tint in a sunny pond-style tank.
Early warning: white cloudiness in an established tank, especially after a big feed or filter clean. Test ammonia and nitrite.
Serious: ammonia or nitrite above zero with fish in the tank; dense green water that won't clear (algae blooms can use up oxygen at night).
Emergency: cloudy water with a bad smell, a missing or dead fish, or fish gasping at the surface. Act now.

What to do right now

Count your fish. If one is missing, search behind rocks, in caves and around the filter. Remove any dead fish and rotting food at once.
Test ammonia, nitrite and pH. This tells you whether the cloudiness is harmless or dangerous.
If ammonia or nitrite is above zero, or the water smells: do a 30–50% water change with dechlorinated, temperature-matched water, add an air stone, and follow the ammonia spike steps.
If readings are zero and the fish are fine: don't chase it. Feed lightly and wait.
Stop or cut feeding for a day or two. Food is fuel for the bloom.
Add aeration if fish are near the surface. Blooms use oxygen.

What to do next

White bacterial bloom: patience

Aquarium Co-op suggests waiting one to two weeks for the bacteria to rebalance. Keep feeding light, keep testing ammonia and nitrite, and do normal-sized water changes only if readings rise.

What NOT to do in a new tank:

Don't keep changing all the water. Huge daily water changes remove the food the good bacteria need and can drag the bloom out. Aquarium Co-op lists many large water changes as a mistake here.
Don't scrub or replace the filter. It holds the bacteria that will eventually clear the bloom. Over-cleaning resets the clock.
Don't add lots of fish to "use up" the bloom.
Don't dose chemicals to make the water look clear while ammonia is above zero. Fix the cause first.

Particles and dust

Let the filter catch them; add a fine filter pad for a few days and rinse or replace it as it clogs.
Vacuum the gravel during regular water changes.
Rinse new gravel and sand thoroughly before adding it next time.
Feed foods that hold together, and only what's eaten in a couple of minutes.
A water clarifier (which clumps fine particles so the filter can catch them) can help, but follow the label and keep shrimp in mind.

Green water

Green water often regrows faster than water changes can remove it. A UV steriliser, which exposes water to ultraviolet light as it passes through a small unit, is the most reliable fix. Also cut the lights to 6–8 hours, keep the tank out of sun, and feed less. Be careful with long blackouts: Aquarium Co-op warns that a mass die-off of algae can cause an ammonia spike. The Wikipedia overview notes green water is common in new tanks. More in the algae guide.

Yellow-brown tannins

No treatment is needed for the fish. If you prefer clear water: soak or boil new wood before adding it, do regular water changes, and run activated carbon or another chemical filter media in the filter. Carbon fills up and must be replaced. Remove carbon during any medicine treatment, because it absorbs medicine too.

Rot blooms after a death or overfeed

Treat as an ammonia emergency: remove the source, change 30–50% of the water, aerate, test daily, and repeat until ammonia and nitrite stay at zero. UF/IFAS lists decaying uneaten feed and organic matter as ammonia sources (UF/IFAS), and the Merck Veterinary Manual lists overfeeding and dying algal blooms among common causes of low oxygen. See overfeeding.

Cautions

Shrimp and snails are sensitive to ammonia and low oxygen; they're often the first to die in a rot bloom.
Plecos, other catfish and loaches live on the bottom, where rotting waste collects. Watch them closely.
Live plants are a help: they compete with algae and bacteria for nutrients. Remove any that are melting.

Products that help

Liquid ammonia and nitrite test kits: tell you in minutes whether cloudiness is harmless or dangerous.
Gravel vacuum (siphon): removes trapped food and waste during water changes.
Water conditioner that detoxifies ammonia: buys time during a rot bloom.
Air stone and air pump: keeps oxygen up while a bloom uses it.
Fine filter pad or floss: catches dust and particles; rinse or replace it often.
UV steriliser: the most reliable cure for green water.
Activated carbon: removes tannin colour and some odours; replace when spent, remove during medication.
Water clarifier: clumps fine particles so the filter can catch them; a cosmetic helper, not a cure.

Keep on hand: ammonia and nitrite tests, conditioner, and an air stone.

How to prevent it

Cycle new tanks before adding fish, and add fish gradually.
Feed small amounts and remove leftovers.
Rinse new gravel and soak new wood before use.
Clean filters gently in tank water, never all the media at once.
Keep lights on a 6–8 hour timer and the tank out of direct sun.
Count your fish every day.

Why it happened

A new tank still cycling, too much food, a dead fish or snail out of sight, an over-cleaned filter, dusty new gravel, too much light, or new driftwood.

When to get help

Ask a good local fish store if cloudiness lasts more than three weeks, if ammonia won't come down, or if fish keep dying. Bring a water sample in a clean jar. A vet who sees fish is the right call if fish show sores, fin damage or odd behaviour after the bloom. Honest limit: cloudy water itself rarely kills fish, but what's causing it sometimes does.

In the game

Cloudy water isn't a card in Multiple Tank Syndrome. The related cards are Ammonia spike ("a bacterial bloom after overfeeding") and Algae takeover.