Green Water (Free-Floating Algae)
Cloudy green water from free-floating single-celled algae -- usually a light/nutrient imbalance, often triggered by too much direct light.
Runner-up: Brown/Diatom Algae · Retake the quiz
Why this fits you
- Your water looks cloudy or tinted green, not clear with algae growing on surfaces.
- Your tank gets direct sunlight or runs its light for long hours.
- This started relatively suddenly rather than building up slowly.
Green water is caused by free-floating single-celled algae suspended throughout the water, usually triggered by excess light (often direct sunlight hitting the tank) combined with available nutrients. Unlike algae growing on surfaces, this type genuinely clouds the water itself.
How it compares
| Algae type | Appearance | Typical cause |
| Green Water | Cloudy green water | Excess light, often direct sun |
| Brown/Diatom Algae | Brown dust on surfaces | Silicates in new tanks |
| Green Spot/Hair Algae | Green spots or strands | Light/nutrient imbalance |
| Black Beard Algae | Dark bristly tufts | Unstable CO2/flow |
Buying checklist
- Reduce light duration and intensity, and block any direct sunlight reaching the tank
- A UV sterilizer is an effective, fairly fast fix if the problem persists
- Don't overcorrect by blacking out the tank for days -- reduce light, don't eliminate it
When this isn’t the right fit
Green water does not fit if the water is clear and the glass and plants are coated. A tank blacked out for days can harm live plants, so reduce light and block sunlight without eliminating it. If the water is cloudy white or grey rather than green, it is probably a bacterial bloom, which has a different cause and often clears on its own.
Other results from this quiz may suit your situation better:
- Brown/Diatom Algae: A brownish dusty coating, most common in newer tanks
- Green Spot/Hair Algae: Hard green dots or stringy green growth
- Black Beard Algae (BBA): Dark, bristly tufts, often on plant edges or hardscape
If your fish or plants show signs of stress or illness, test your water and ask an experienced fishkeeper or an aquatic veterinarian before you treat.
Run the numbers
Frequently asked questions
Will a water change fix green water?
Usually not on its own -- a water change dilutes it temporarily, but the algae will typically bloom back unless the underlying light/nutrient imbalance is addressed.
How long does green water take to clear up?
With reduced light and addressed nutrients, often 1-2 weeks; a UV sterilizer can clear it much faster, often within days.
Is green water harmful to fish?
Not directly harmful in most cases, though it can indicate a light/nutrient imbalance worth addressing, and severely reduces visibility in the tank.
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