Why is my aquarium water cloudy?

Cloudy water has a cause. The longer troubleshooting note is cloudy aquarium water. This is the version you can use standing in front of the tank.

Test ammonia first. If it is above 0 and there are fish in the glass, that is the problem. Cloud color is the clue for everything else.

White / milky, especially on a new tank

Bacterial bloom. The tank is cycling, you overfed, or you rinsed the filter in the sink. Free-floating bacteria explode when dissolved organics spike.

Do not empty the tank. Do not replace all the filter media. Feed less. Wait. Let the filter catch up. Cycling. Overfeeding.

A UV sterilizer can clear the look faster. It does not finish a cycle.

Dusty after you scaped

Sand or aquasoil fines. It settles. Filter floss speeds it up. Not an emergency unless you also have ammonia.

Rinse sand in a bucket until the water runs clearer before it goes in next time. New aquasoil often clouds for a day. That is dust, not a crash.

Green

Algae in the water column: too much light, too many nutrients, not enough plants winning. Cut the photoperiod by an hour or two for two weeks. Algae control. Light schedule.

Green glass film is not green water. Wipe the glass. Green soup is the water itself.

Brown film on glass and sand in a new tank

Diatoms. They like silicates and new tanks. They fade as the tank matures. Wipe, do not bleach the scape.

Fish gasping in cloudy water

Not cosmetic. Test ammonia now. Increase surface agitation. Gasping.

What not to do

• Dump “clear water” slime that coats gills.
• 90% water change plus a new filter cartridge plus new fish.
• Ignore it because the shop said cloudy is normal forever. New-tank bloom is common. Forever bloom is overfeeding or a filter you keep sterilizing.

If nitrates were already high, do a measured change: water change calculator. Conditioner on the new water only.