đ Green Water (Algae Bloom)
Green water is caused by a bloom of free-floating microscopic algae (phytoplankton) that turns aquarium water pea-soup green. While unsightly and obscuring your view, green water is actually harmless to fish and often indicates excellent water conditions for life - just not the life you want!
đ Quick Facts
| Type | Free-floating phytoplankton bloom |
|---|---|
| Appearance | Green, cloudy water (can't see through) |
| Severity | Moderate - Harmless but unsightly |
| Common Causes | Excess light, high nutrients, new tank |
| Treatment | UV sterilizer, blackout, daphnia |
đ Identification
- Green tint - Water itself is colored green
- Cloudy/hazy - Reduced visibility
- Severity varies - Light tint to opaque pea soup
- Doesn't settle - Algae stays suspended
- Different from green on surfaces - This is in the water column
â ī¸ Common Causes
- Excess light - Direct sunlight or too-long photoperiod
- High nutrients - Elevated nitrates, phosphates
- New tank syndrome - Unbalanced ecosystem
- Overfeeding - Excess nutrients fuel growth
- Recent disturbance - Substrate disruption releases nutrients
- Decaying matter - Dead plant material, uneaten food
đ Treatment Options
1. UV Sterilizer (Most Effective)
- UV light kills free-floating algae
- Clears water in 3-7 days
- Doesn't address root cause (algae may return)
- Safe for fish, plants, beneficial bacteria
- Can be used temporarily then removed
- Properly sized for tank volume
2. Complete Blackout
- Cover tank completely for 3-4 days
- No light = no photosynthesis = algae dies
- Keep filter running
- Fish are fine without light briefly
- May stress some plants
- Address causes or it returns
3. Daphnia (Biological Control)
- Daphnia (water fleas) eat phytoplankton
- Add live daphnia culture to tank
- They multiply and consume algae
- Fish will eat daphnia (that's fine - dual purpose!)
- Natural and interesting solution
- May need repeated additions
4. Fine Mechanical Filtration
- Polishing pads or filter floss
- Diatom filters (very fine)
- Change frequently as they clog
- Helps but slow as sole method
5. Water Changes
- Large water changes temporarily dilute
- Won't solve alone (algae multiplies back)
- Good to combine with other methods
- Removes excess nutrients
6. Flocculants/Clarifiers
- Clump algae for filter removal
- Quick cosmetic fix
- Doesn't address cause
- Can stress fish if overused
đĄī¸ Prevention
- Avoid direct sunlight - Position tank away from windows
- Limit photoperiod - 6-8 hours for problem tanks
- Don't overfeed - Major nutrient source
- Regular maintenance - Water changes, gravel vacuum
- Live plants - Compete for nutrients
- Proper stocking - Don't overstock
âąī¸ Treatment Timeline
| Method | Time to Clear |
|---|---|
| UV Sterilizer | 3-7 days |
| Blackout | 3-4 days |
| Daphnia | 1-2 weeks |
| Fine filtration alone | 1-3 weeks |
â Common Questions
Is green water harmful to fish?
No! Green water is actually excellent for fish. The algae produces oxygen and can even be beneficial for fry. It's purely an aesthetic issue for the aquarist.
Why does it keep coming back?
You're treating symptoms but not causes. If you have excess light and nutrients, the conditions for algae blooms remain. Address the underlying factors.
Will it go away on its own?
Sometimes, as the ecosystem balances. But it can persist for weeks or months. Active treatment is faster and more reliable.
đ Summary
Green water is frustrating but harmless. UV sterilization is the fastest and most reliable solution, clearing water in under a week. A complete blackout also works well. For ongoing prevention, control light exposure and nutrient levels. Live plants help compete with algae for nutrients. While green water can seem alarming, it's one of the easier algae problems to solve - your fish are perfectly happy even if you can't see them!