The short answer
Green water is a bloom of microscopic, free-floating algae — millions of single cells suspended in the water, turning it pea-soup green. Unlike algae on glass or plants, you can’t wipe it off. The two things that actually work are a 3–4 day blackout or a UV clarifier, followed by cutting the light and nutrients that triggered the bloom. It looks alarming but it’s harmless to fish, and very fixable.
Clear the bloom
You have two reliable options to physically remove the floating algae:
- Blackout. Turn the lights off and cover the tank completely with towels or a blanket for 3 to 4 days. Without light the suspended algae die off. Fish and most plants tolerate the darkness fine; feed lightly and do a big water change afterwards.
- UV clarifier. A UV steriliser plumbed into your filter flow zaps free-floating algae as water passes through, clearing the tank in a few days without a blackout. It’s the go-to if green water keeps recurring.
Fix what caused it
Once the water is clear, deal with the trigger or the bloom comes straight back.
- Cut the light. Excess light — especially direct sunlight hitting the tank — is the classic cause. Move the tank out of sunlight and keep your photoperiod to 6–8 hours on a timer.
- Reduce nutrients. Overfeeding, overstocking and skipped water changes let nitrate and phosphate build up and feed the bloom. Feed less and keep up weekly 25–30% water changes.
- Grow more plants. Healthy fast-growing plants out-compete floating algae for the same nutrients.
Keep it clear
Green water thrives on surplus light and nutrients, so keep both in check: modest photoperiod on a timer, no sunlight on the glass, sensible feeding and consistent water changes. For related help, see why you have so much algae and our how to get rid of aquarium algae guide.