Cloudy aquarium water: white haze, green water and tannins

Start with the color

• white or milky with no visible specks – a bacterial bloom, bacteria floating in the water,
• gray with fine specks that slowly settle – substrate dust or debris,
• green like pea soup – free-floating algae,
• yellow to brown like tea – tannins from wood or leaves.

Each one has a different cause and a different fix, so work out which one you have first.

Milky water in a new tank

A milky haze in the first days or weeks is a bacterial bloom. There are plenty of nutrients in the water and the bacteria settling in the filter and substrate cannot keep up yet, so free-floating bacteria multiply instead. It usually clears by itself within one to two weeks.

The hard part is leaving it alone:

• do not clean or replace the filter media – the bacteria you need are only just settling in,
• skip extra water changes done only to clear the haze – it looks better for a day, then the bacteria multiply again; keep the regular changes your cycling routine needs,
• skip clarifiers and other products sold for clear water – they do not fix the cause,
• feed lightly – leftover food is more fuel for the bloom.

Test NH₄ and NO₂. The haze itself does not make the water toxic, but it means the tank is still cycling. Add fish once both read zero for several days in a row, as described in the guide Cycling a planted aquarium. If fish or shrimp are already in the tank and NH₄ or NO₂ is above zero, change about half the water right away with conditioned water at the same temperature and test daily – here the water change comes before clarity.

If the haze lasts longer than two or three weeks, look for a steady source of waste: overfeeding, a dead animal, rotting plants or too many fish for a young filter.

Cloudy water in an established tank

When a tank that has been clear for months suddenly turns cloudy, there is usually a trigger: overfeeding, a dead fish or rotting plants, replanting or stirred-up substrate, a filter cleaned too thoroughly, or a large group of new fish added at once. Either there is a surplus of nutrients, or part of the bacteria that used to handle them is gone. Haze right after a water change or cleaning is usually just debris stirred up from the bottom, and it settles on its own.

• find and remove the cause – a dead fish, uneaten food, melting leaves,
• test NH₄ and NO₂; if they are above zero, change about half the water with conditioned water at the same temperature and test again; repeat the next day if the readings are still above zero,
• rinse filter media only in tank water and never all of it at once.

Bacteria in the water use up oxygen. If fish hang at the surface or breathe fast, point the filter outlet so it ripples the surface or add an air stone. Warm water holds less oxygen. In a CO₂ tank, too much CO₂ causes the same signs – switch it off until the fish recover, then run it at a lower rate.

Green water

Green water is single-celled algae suspended in the water column. It is driven mostly by too much light combined with ammonia and organic waste – from overfeeding, disturbed substrate or an overdose of fertilizer. In a new tank, ammonia from cycling is often the trigger. Direct sunlight from a window is a common culprit too.

Water changes alone do not fix it – the algae multiply faster than you can remove them, and fine filter floss cannot catch them.

• a UV sterilizer stops the algae from reproducing and usually clears the water within a few days,
• a blackout – cover the tank completely with the lights off – also works, but it weakens plants, so treat it as the second choice,
• after UV or a blackout, test NH₄ and change about half the water with conditioned water at the same temperature, because dying algae release ammonia,
• cut the photoperiod to 6–8 hours, shade the tank from the sun and add fast-growing stems and floating plants.

During a blackout keep the filter running, switch CO₂ off and add aeration – plants do not use CO₂ in the dark, so it would build up. Keep the blackout to three to five days; longer than that hits plants hard, and if the water is still green a UV sterilizer is gentler than extending it. Feed only very lightly while it lasts.

UV and blackouts remove the algae, not the reason they grew. Without changes to light and the source of waste the green comes back. Do not stop your regular fertilizing because of it. Algaecides do not address the cause either, and they have no place in a shrimp tank.

Yellow or brown water: tannins

A tea-colored tint comes from tannins released by driftwood and leaves. They do not harm fish or shrimp and lower pH slightly; some keepers of soft-water fish like the look. Removing them is a matter of taste.

• activated carbon or a synthetic adsorbent in the filter removes the color fastest; carbon saturates over time and has to be replaced,
• regular water changes clear it gradually,
• soak or boil new wood before it goes in – it releases part of the tannins in advance, though never all of them.

Dust after setup

Haze right after filling is fine particles from the substrate and decor. You can see the specks, and they usually settle or end up in the filter within a few days. Fine filter floss or a polishing pad speeds it up; replace it once it has caught the dust.

Aquasoil stirs up easily. Place rocks and wood first, then pour slowly onto a plate or a plastic sheet laid on the soil. If the water is very cloudy after filling, drain it, refill and start the filter only once the water is clear.

What to log

Cloudiness rarely comes out of nowhere. Alongside water changes, log filter cleaning, new fish and plants, new wood and changes in feeding or lighting. When the water turns cloudy you can see what changed a few days before, and the tank's age tells you whether it is still cycling or something shifted in an established tank.

NH₄ and NO₂ readings help, and so does a photo from the same angle every few days – clarity is easier to compare in photos than from memory. The Advisor sees the tank's day and these entries, so its answer about cloudy water starts from what actually happened in the tank.