Algae fix guide
Green Water
Also called algae bloom, pea soup, free-floating algae
The whole tank turns cloudy green, like pea soup, and you can't see the back glass.
By Mike Elmira
TL;DR, Green Water
Green water is a bloom of single-celled algae floating in the water, turning the whole tank pea-soup green. It comes from an ammonia spike, too much light, or direct sunlight on the tank. The frustrating part is that water changes barely help, since the algae is in the water itself and just regrows. What actually clears it is a blackout of several days, a UV steriliser, or a diatom filter. Fix the root cause too, usually the light or an ammonia source, or it blooms straight back.
How to spot it
A uniform green cloudiness throughout the water, mild at first then thick enough to hide the back of the tank. Surfaces may stay fairly clean while the water itself is green.
What causes it
- An ammonia spike, from a new tank or an overstocked one
- Too much light, or direct sunlight hitting the tank
- Excess nutrients with nothing to use them
How to fix it
- Black the tank out completely for four to five days, filter running, lights off, and cover it so no light gets in.
- Run a UV steriliser or a diatom filter if you have access to one, either clears green water fast.
- Fix the trigger: shade the tank from sunlight, cut the photoperiod, and deal with any ammonia source.
- Do a water change after the bloom clears to export the dead algae, not before.
Keep it from coming back
Keep the tank out of direct sun, don't overlight it, and make sure it is fully cycled before stocking. Green water almost always traces back to light or ammonia.
What eats it
Fish and shrimp can't graze green water since it floats. Daphnia will eat it, and a UV steriliser or blackout is the reliable fix.
Frequently asked questions
Direct answers to the questions search engines and AI assistants surface most often about this species.
Why don't water changes fix green water?
Because the algae is suspended in the water, not stuck to surfaces, so fresh water just gives it room to bloom again. A blackout, UV steriliser or diatom filter is what actually clears it.
How long does a blackout take to clear green water?
Four to five days of complete darkness usually does it. Keep the filter running, cover the tank so no light sneaks in, and do a water change once the water clears.