Cloudy water

First: what kind of cloudy?

Cloudy water isn't just one thing. The color says a lot about the cause:

• White or milky: usually bacteria.
• Green: free-floating algae.
• Gray or brown with particles: dust from the substrate or debris.
• Yellow or brown, but clear: tannins from driftwood or leaves, which are harmless.

A white haze in a new tank

The most common case is water turning milky white a few days or weeks after the tank is set up. This is a bacterial bloom: free-floating bacteria multiplying on nutrients in the water before the bacteria in the filter and substrate have taken over.

It's part of cycling, and usually clears up on its own within a few days, sometimes a bit longer. See Cycling: the nitrogen cycle explained.

• Don't replace all the water or wash the filter. That sets the cycling back.
• Feed lightly, so the bacteria don't get more to live on.
• Test for ammonia and nitrite. If there are fish in the tank, do a water change if either is above 0.

A white haze can also show up in a tank that has been running for a long time. Then it's often a sign of a sudden load of waste: overfeeding, a dead fish or a filter that has been cleaned too thoroughly.

Green water

Green water is caused by free-floating algae, tiny algae that live in the water instead of on the glass and plants. They multiply very quickly when there's plenty of light and nutrients.

• Check the light. Is it on for too long, or does sunlight reach the tank? See Lighting: how long and how bright.
• Feed less and change the water. That takes nutrients away from the algae. But water changes alone often help only for a while, since the algae multiply quickly again.
• Black out the tank for a few days, with a blanket or cardboard around it. The algae need light, and the fish can handle a few days in the dark. Do a water change afterward.
• A UV sterilizer kills the floating algae that pass through it, and is the surest fix if green water keeps coming back.

There's more about algae in Algae: which kind and what to do.

Gray or murky

Particles floating in the water usually come from:

• Substrate that wasn't rinsed well enough, especially in a new tank.
• Fish that dig, such as loaches and cichlids.
• A water change that stirred up the substrate.
• A filter that's too small or clogged.

Dust from the substrate often settles within a day or two. If it keeps coming back, check whether the filter has enough capacity and whether the filter media is clogged. Fine filter floss catches small particles.

Yellow or brown water

Driftwood, alder cones, oak leaves and Indian almond leaves release tannins that tint the water yellow or brown, like tea. It's harmless, and many fish from South America and Asia live in water like this in the wild. If you don't like it, water changes and activated carbon in the filter help, and driftwood gradually stops coloring the water as much.

Overfeeding is the most common cause

What most kinds of cloudy water have in common is that too much food makes them worse. Food that isn't eaten rots and feeds both bacteria and algae. See Feeding: less is more.

Water clarifiers

There are products that make particles clump together so the filter can catch them. They can help with dust, but they don't fix the cause of bacteria or algae. Find the cause first.