Why Is My Aquarium Water Green?
The Tank Guide · A FishKeepingLifeCo publication — Aug 2026
Green aquarium water is usually a bloom of microscopic algae suspended in the water column. Unlike algae attached to glass, it colors a jar of tank water green and cannot simply be wiped away.
Confirm green water
Fill a clear glass and hold it against white paper. Green-tinted water indicates suspended algae. If the glass sample is clear but the aquarium still looks green, wipe the viewing pane: attached film algae may be the real cause.
Why it appears
Long or intense lighting, direct window sun, and excess available nutrients can combine faster than plants and routine maintenance can compensate. A bloom does not prove that one particular nutrient test is high; algae can consume nutrients while it grows.
A practical treatment sequence
- Keep the tank out of direct sunlight and put aquarium lights on a timer.
- Remove decaying material, avoid overfeeding, and maintain regular partial water changes.
- Use healthy plants to compete for light and nutrients.
- For a severe bloom, a short blackout can reduce algae; maintain aeration and monitor livestock.
- A correctly sized UV sterilizer can clear suspended algae, but it does not correct the light or nutrient imbalance that caused it.
Related cloudy-water diagnoses
White-gray haze is more consistent with an aquarium bacterial bloom. Clear tea-colored water suggests tannins. Return to the cloudy aquarium water hub for the complete color-by-color check.
Green aquarium water is usually a bloom of microscopic algae suspended in the water column. Unlike algae attached to glass, it colors a jar of tank water green and cannot simply be wiped away.
Confirm green water
Fill a clear glass and hold it against white paper. Green-tinted water indicates suspended algae. If the glass sample is clear but the aquarium still looks green, wipe the viewing pane: attached film algae may be the real cause.
Why it appears
Long or intense lighting, direct window sun, and excess available nutrients can combine faster than plants and routine maintenance can compensate. A bloom does not prove that one particular nutrient test is high; algae can consume nutrients while it grows.
A practical treatment sequence
- Keep the tank out of direct sunlight and put aquarium lights on a timer.
- Remove decaying material, avoid overfeeding, and maintain regular partial water changes.
- Use healthy plants to compete for light and nutrients.
- For a severe bloom, a short blackout can reduce algae; maintain aeration and monitor livestock.
- A correctly sized UV sterilizer can clear suspended algae, but it does not correct the light or nutrient imbalance that caused it.
Related cloudy-water diagnoses
White-gray haze is more consistent with an aquarium bacterial bloom. Clear tea-colored water suggests tannins. Return to the cloudy aquarium water hub for the complete color-by-color check.