Cloudy aquarium water: causes and fixes

Cloudy water is one of the most common beginner worries. The color tells you most of what you need to know, and most causes are easy to fix.

First, what color is it?

• White or gray haze: usually a bacteria bloom, or dust from new gravel.
• Green: free-floating algae.
• Yellow or brown, like weak tea: tannins from driftwood or leaves.

White or gray cloudy water

In a new tank: a bacteria bloom

In the first weeks, bacteria multiply quickly in the water before settling into the filter and gravel. The water turns milky, often within a day or two of setup or of adding fish. It's usually harmless and clears by itself within a few days to a couple of weeks.

What to do: be patient. Feed lightly, don't clean or replace the filter, and keep testing ammonia and nitrite if fish are in the tank. Avoid big water changes just to clear the cloudiness, since they can make the bloom last longer.

Right after setup: substrate dust

Unrinsed gravel or sand leaves fine dust in the water. The filter clears it in a day or two. Next time, rinse new substrate in a bucket until the water runs mostly clear.

In an established tank: too much food or waste

If a tank that's been clear for months suddenly goes cloudy, something is usually breaking down: uneaten food, a dead fish or snail, or rotting plants. This can come with an ammonia spike.

What to do: test ammonia and nitrite. Look for anything dead or rotting, vacuum the gravel, change 25–50% of the water, and cut back on food.

Green cloudy water

Green water is tiny algae floating in the water. It's caused by too much light, often sunlight from a nearby window, combined with leftover nutrients from food and fish waste.

What to do:

• Cut the light to 6–8 hours a day, and keep the tank out of direct sunlight.
• Feed less and keep up weekly water changes.
• Add fast-growing or floating plants to compete for nutrients.
• For stubborn cases, a UV sterilizer clears green water reliably.

Water changes alone rarely fix green water, because the algae grow back quickly. Fixing the light is the key.

Yellow or brown water

Driftwood, catappa leaves and some plant roots release tannins that tint the water like tea. It's harmless, and many fish from soft-water rivers like it.

What to do if you don't like the look: soak or boil new driftwood before adding it, do regular water changes, or add activated carbon to the filter, which removes the tint.

Should I use a water clarifier?

Clarifiers clump tiny particles together so the filter can catch them. They can help with dust, but they don't fix the cause of a bacteria bloom, algae or rotting food. Sort out the cause first.

Related: How to cycle a new fish tank · Water change calculator