This is part of our full algae identification and treatment series — start there if you’re not sure which algae you’re actually looking at.
What it looks like
The whole water column turns cloudy green — anywhere from a faint tint to a genuine “pea soup” you can’t see through. Unlike every other algae on this list, green water isn’t attached to anything; it’s a population of free-floating single-celled algae suspended directly in the water.
Why it happens
Green water is a bloom event, not a slow buildup — it needs a sudden spike in both light and nutrients to take off, and it usually appears within a day or two of that trigger. Common causes:
- Direct sunlight hitting the tank, even for part of the day. This is the single most common cause — a tank near a window that gets a few hours of direct sun is far more prone to blooms than one under controlled artificial light.
- A large nutrient spike — a big feeding, a fish loss that went unnoticed for a day, or overhauling the substrate — that hands the free-floating algae a sudden abundance to multiply into.
- New tanks with light running but no plant mass yet established to absorb the same nutrients.
How to clear it
- Blackout. Cover the tank completely for 3–4 days — no light at all, feed sparingly. Without light, the free-floating algae population collapses quickly since it can’t photosynthesize.
- Cut off direct sunlight permanently if that’s the trigger — reposition the tank or add a curtain/blind. This is the fix that actually prevents recurrence; a blackout alone just resets a bloom that will return if the light source doesn’t change.
- A diatom filter or UV sterilizer clears a bloom in hours instead of days if you already have one, but neither is required — the blackout method works reliably on its own.
- Get fast-growing plants and floaters established so the tank has permanent competition for the same nutrients a bloom needs. A dense floating layer is particularly effective here since it directly competes with suspended algae for the same light and nutrient pool — see our guide to floating plants. I’ve had greenwater get resolved and never return after letting floating plants completly take over the top of my tank.
- Shortcuts: UV filters or copper foses can quickly remove and kill off the greenwater algae. Just make sure to keep changing your filter sponges in order to remove the dead algae from the water column!
Green water looks alarming but is rarely dangerous to fish on its own — it’s a nutrient and light imbalance, not a toxin. Resolve the underlying trigger and it typically doesn’t return. Some people even like keeping it in their tank in order to keep nutrients low!
