The short answer
Green water is a bloom of microscopic free-floating algae, triggered by too much light and too many nutrients. It’s the tank telling you the balance between light, nutrients and plant growth has tipped. To fix it you starve the algae: reduce lighting, cut the nutrients feeding it, and physically remove the algae with a blackout or UV clarifier. Water changes help but rarely fix it alone.
What fuels it
Two things drive a green bloom, and usually both are present:
- Too much light — long lighting periods, a tank near a window, or lights that are too strong for the plant load.
- Excess nutrients — nitrate and phosphate from overfeeding, overstocking or skipped water changes.
Because the algae floats and multiplies fast, diluting it with water changes is like bailing a leaky boat. You have to remove the source. See our guide on how to get rid of aquarium algae for the full playbook.
How to clear it
- Cut the light. Reduce your photoperiod to around 6–7 hours and move the tank out of direct sunlight. A total 3–4 day blackout (cover the tank completely) starves floating algae quickly.
- Reduce nutrients. Feed less, don’t overstock, and do regular water changes to export nitrate. See how to lower nitrates.
- Add live plants. Fast growers outcompete algae for the same nutrients, which is why heavily planted tanks rarely go green.
Keep it from coming back
Green water is a symptom of imbalance, so prevention is the same as the cure: a sensible photoperiod, restrained feeding, healthy plants and consistent maintenance. Test your nitrate and phosphate to confirm nutrients are under control, keep up with routine maintenance, and the tank should stay clear. If algae keeps returning on surfaces too, revisit our cloudy water answer to rule out other causes.