Cloudy aquarium water: causes by colour and how to clear it
AquaLog guide · Published
In short: The colour tells you the cause. A white or grey haze is usually dust from new substrate or a bloom of free-floating bacteria, green water is microscopic algae, and yellow or brown water is tannins from wood or leaves or a build-up of waste. If ammonia and nitrite read 0 ppm and the fish behave normally, most cloudiness is not an emergency and clears with steady maintenance rather than chemicals.
What does each colour mean?
| Colour | Most likely cause | Harmful? | First step |
|---|---|---|---|
| White or grey, right after setting up or adding substrate | Fine dust from gravel or sand | No | Let the filter clear it |
| Milky white, in a new tank, or after overfeeding or a deep clean | Usually a bacterial bloom | It can come with ammonia or nitrite | Test ammonia and nitrite; feed less |
| Green, like pea soup | Free-floating algae (green water) | Not directly; it can lower oxygen at night | Cut the light and the feeding |
| Clear yellow or tea-brown, after adding wood or leaves | Tannins | Not usually | Leave it, or lighten it with water changes |
| Dull yellow in an older tank, with high nitrate | Dissolved waste building up | Yes, over time | More or larger water changes |
| Grey and smelly, suddenly | Something rotting: a dead fish or snail, or a lot of uneaten food | Yes | Find and remove it, then test and change water |
Whatever the colour, test ammonia and nitrite first. If either reads above zero, that is the real problem and the cloudiness is a symptom. See how to lower ammonia in an aquarium.
White or grey: is it dust or bacteria?
Dust shows up the moment you fill a new tank or add substrate, and it is worst with unwashed gravel or sand. It is harmless. Run the filter and give it time; fine filter floss can help it clear. Next time, rinse new substrate until the water runs clear, as described in how to set up your first freshwater aquarium.
A milky haze that appears in a new tank, or after a dead fish, heavy overfeeding or a deep clean, is usually called a bacterial bloom: free-floating bacteria multiplying on waste dissolved in the water. Whatever the exact cause, it tells you there is more waste in the water than the tank is handling. In a new tank that often means it is still cycling, so test ammonia and nitrite.
- Feed lightly until it clears.
- Leave the filter alone. Don't replace or scrub the media; the bacteria you need live there.
- Keep up your normal partial water changes, and change more if ammonia or nitrite climb.
- If ammonia and nitrite stay at zero and the fish behave normally, keep to your routine and give it time. If either reading rises or the fish change their behaviour, treat it as a water-quality problem.
Green water: what causes it?
Green water is a bloom of microscopic algae floating in the water rather than growing on the glass. Like all algae, it grows on light and nutrients, mainly nitrogen and phosphorus, which in an aquarium come from fish food and waste. A tank in direct sunlight, with the light on for most of the day, or with high nitrate is the classic case.
- Move the tank out of direct sun, or block the sunlight.
- Shorten the time the light is on, and use a timer so it is the same every day.
- Feed less and test nitrate. If it is high, step up your water changes; see how to lower nitrate.
- Add fast-growing or floating plants. They take up nitrate, a plant nutrient, leaving less for the algae.
- Some keepers fit an ultraviolet (UV) clarifier for stubborn green water. It only treats the water pumped through it and does nothing about the light and nutrients behind the bloom, so fix those first.
Water changes help, but if the light and nutrients stay the same the algae can grow back.
Green water is not poisonous, but it changes oxygen levels. Algae make oxygen in the light and use it in the dark, so oxygen is lowest just before dawn, and algae that die off use up oxygen as they rot. University of Florida (UF/IFAS) fish specialists describe the same daily cycle in ponds. If your fish gasp in the early morning, read why fish gasp at the surface.
Yellow or brown: tannins or waste?
Tannins are natural plant compounds that leach from driftwood, dried leaves and other plant matter and stain the water the colour of tea. The colour on its own does not mean the water is dirty, though a nitrate test is the way to be sure (see below). Several popular fish come from tannin-stained water in the wild: our care guide describes the natural habitat of neon tetras as soft, acidic, tannin-stained water, and of harlequin rasboras as tannin-rich.
If you prefer clear water, soak new wood in a bucket before use and change its water as it darkens, so less colour reaches the tank. Regular water changes dilute the colour, and activated carbon in the filter can help lighten it.
Waste gives a dull yellow tint in tanks that are overdue a water change, together with rising nitrate. Test nitrate to tell the two apart. If it is above about 20–40 ppm, your water change schedule needs adjusting.
When is cloudy water a problem?
- Ammonia or nitrite reads above 0 ppm.
- Fish are gasping at the surface, hiding, not eating or clamping their fins.
- The water smells rotten.
- An established tank turns cloudy suddenly. Count your fish and check the filter is running.
- The cloudiness appeared after you added a medicine or water treatment. Read the label and test the water.
In any of these cases, test, do a partial water change with dechlorinated water at the same temperature, and look for the cause.
How do you keep the water clear?
- Rinse new substrate well before it goes in.
- Feed only what the fish finish in a couple of minutes.
- Stock slowly and within the tank's limits. Check adult sizes in the care guide.
- Change 10–25% of the water every week and vacuum the substrate.
- Rinse filter media in old tank water, never under the tap, and never all of it at once.
- Keep the tank out of direct sunlight.
If the water keeps clouding over, a record helps you find the pattern. AquaLog logs each water test, water change and filter clean per tank, so you can compare those dates with when the cloudiness started.