Green algae
Green Water in Aquarium
A bloom of microscopic green algae that turns the water column cloudy green instead of coating surfaces.
Save this guide or add this issue to one of your aquariums.
What to know first
- Green Water turns the water itself green because single-celled algae are suspended in the water column.
- It is not a coating on glass, leaves, or hardscape, even if surfaces also need normal cleaning.
- Excess light, direct sunlight, overfeeding, ammonia traces, and unstable filtration are common triggers.
- Persistent cases often respond well to a correctly sized UV sterilizer while the underlying cause is corrected.
Do you have Green Water?
You probably have Green Water if...
- The aquarium water looks green or pea-soup tinted throughout the aquarium.
- The cloudiness remains visible even after the glass is wiped clean.
- The problem is suspended in the water column, not attached as spots or fuzz.
- It often follows excess light, direct sunlight, overfeeding, ammonia traces, or unstable filtration.
- It looks green rather than white, grey, or oily on the surface.
Key details
- Category
- Green algae
- Growth form
- Suspended bloom
- Main color
- green / yellow green
- Attachment
- Not attached
- Removal difficulty
- moderate
- Main trigger
- Green water blooms when suspended single-celled algae take over the water column in unstable conditions.
- Most affected area
- Water column
Detailed guide to Green Water
Green Water is a suspended algae bloom. The water itself turns green because tiny free-floating algae multiply in the water column.
This is different from green dust on glass or green film on leaves. Wiping the glass may improve the view slightly, but it will not remove a true suspended bloom.
Common causes
| What you see | Likely cause | What to check first |
|---|---|---|
| Water looks green throughout the aquarium | Suspended single-celled algae | Light exposure, direct sunlight, ammonia traces |
| Bloom follows overfeeding | Extra dissolved organic compounds and nutrients | Feeding amount, uneaten food, filtration |
| Bloom keeps returning | Light and stability are still favoring suspended algae | Photoperiod, sunlight, UV option, water quality |
How to Remove Green Water
Reduce excess light, keep filtration stable, avoid overfeeding, and test for ammonia or nitrite. For persistent blooms, a correctly sized UV sterilizer can clear suspended algae very effectively while the underlying cause is corrected.
How to Prevent Green Water
Avoid direct sunlight, keep stocking and feeding reasonable, let new aquarium biology mature, and do not run intense light over weak plant growth or unstable filtration.
Fix Plan
Today
Check water clarity, reduce feeding, keep filtration running, and improve oxygen exchange at the surface.
This Week
Limit excess light, check for ammonia traces, and consider UV sterilization for persistent green suspended blooms.
Long-Term Prevention
Avoid overfeeding, stock slowly, keep the filter stable, and prevent sudden swings in light or organic load.
Often confused with
Supporting notes
Where you'll usually see it
Throughout the water column rather than attached to surfaces.
Why it shows up
Suspended blooms usually follow excess light, organic waste, ammonia traces, or unstable filtration.
Check this before changing everything
Check water parameters, filtration stability, light exposure, and oxygen exchange.
Common context
Often seen in young, overfed, brightly lit, or recently disturbed aquariums.
Useful tools and lessons
FAQ
Green Water FAQ
What does Green Water look like?
It looks like green or pea-soup cloudy water throughout the aquarium. The glass may be clean while the water itself remains green.
Why does Green Water appear in an aquarium?
It appears when free-floating green algae multiply after excess light, overfeeding, ammonia traces, or filter disruption.
Is Green Water harmful?
The cloudiness itself may be mostly cosmetic, but the cause can affect oxygen and water quality, so livestock behavior and ammonia/nitrite should be checked.
How do you remove Green Water?
Reduce excess light, check ammonia, avoid overfeeding, keep filtration stable, and consider UV sterilization for persistent blooms.
How do you stop Green Water from coming back?
Keep filtration mature, feed lightly, avoid sudden instability, and control light and organic load.