Green water looks dramatic: the tank turns pea-soup green, fish look like they’re swimming in tinted glass, and it feels like you have to “reset everything”. The good news is that green water is usually a straightforward problem — it’s free-floating algae feeding on a combination of light and nutrients. The fix is to reduce one of those inputs while keeping the tank stable.
Why green water happens
- Long light hours: 10–12 hour photoperiods push algae hard.
- Direct sunlight: even a short daily sun beam can trigger blooms.
- Overfeeding: excess waste fuels algae growth.
- New tank instability: blooms are common when the system is still settling.
The fastest safe first steps
- Reduce light: cut to 6 hours for a couple of weeks and block direct sun.
- Feed lighter: reduce portions so less nutrient enters the system.
- Increase water change consistency: smaller regular changes help control nutrient load.
What not to do
- don’t do repeated “100% resets” that destabilise the tank
- don’t add random chemicals without addressing light/nutrients
- don’t scrub everything spotless (you want stability, not sterility)
When blackouts help
A short blackout (2–3 days) can knock algae back, but it works best when you also reduce the underlying drivers. After a blackout, return to a shorter photoperiod and keep feeding controlled, or the bloom will return.
Long-term prevention
Most green water cases disappear when light is sensible, feeding is controlled, and water changes are consistent. Once you stabilise those inputs, the tank becomes much more predictable and algae loses its advantage.
Green water looks bad, but it’s usually an easy fix when you treat it as a light-and-nutrients problem. Keep the system stable, reduce the fuel, and it clears.
