Guide
Green water (algae bloom) fix
Free-floating algae: reduce light and nutrients, water changes.
- Cause: Suspended single-celled algae — usually too much light + nutrients (nitrate/phosphate) together.
- Fast wins: Blackout or reduce photoperiod, increase water changes, and cut back feeding until it clears.
- Tools: Fine filter polishing (temporary), UV steriliser (very effective), or patience plus maintenance.
- Prevention: Stable lighting schedule, regular testing, and avoid overstocking/overfeeding.
What is green water?
The water looks like pea soup — not just a tint on glass. It is free-floating algae, not a film you can wipe. It is common in sunlit rooms or tanks with long photoperiods and high nutrients.
Fix the drivers
Reduce daily light hours (try 6–8 hours on a timer), move the tank away from direct sun, and trim feeding. Test nitrate; if it is high, increase water change frequency until the bloom retreats.
Mechanical removal
Water changes alone can help by diluting algae and nutrients. Some keepers use a temporary UV clarifier, which kills suspended algae as water passes through. Polishing pads can help clear fines after a bloom.
Plants and balance
Healthy fast-growing plants compete for the same nutrients. If you run CO₂ or fertiliser, ensure dosing matches your light — imbalance can fuel algae. See algae 101 and lowering nitrate.
Track nitrate trends and lighting habits with App-aquatic.
Get the free appIs green water dangerous to fish?
Usually not directly, but severe blooms can reduce visibility and slightly alter oxygen dynamics at night. Fix the underlying cause.
Does a blackout work?
Yes — 3–4 days of darkness with a towel over the tank, combined with reduced feeding, often kills free-floating algae. Resume light gradually.
Will it come back?
It can if light and nutrients remain high. Keep testing, stable photoperiods, and sensible stocking.
All guides · Water parameters · Algae 101 · Stocking calculator · Get the app