Troubleshoot
Green Water Bloom: Light vs Nutrients Fix
Green water is a pea-soup bloom in the water column—not film on glass (GDA) and not grey bacterial cloud (cloudy new tank). Fix light hours or use a short blackout/UV; don’t scrape the glass and call it done.
- Green water
- Free-floating bloom
- Blackout / UV
- Light vs nutrients
What you’re seeing
Flashlight test: the beam turns green through the tank. Glass may still wipe clean. Fish are visible but hazy. Grey/white milk is usually bacterial—not this page.
| Pattern | Likely driver |
|---|---|
| Pea soup after raising lights or fertiliser | Light + nutrients with little plant mass |
| New tank, green tint, still cycling | Open niche + long photoperiod |
| Grey milk, not green | Bacterial bloom |
| Only the glass is green film | Green dust |
Likely causes (ranked)
Photoperiod too long for plant mass
Empty water column + 8–10h LEDs.
Quick test: Few plants, strong light, green tint in days.
Nutrient spike (feed / dose / new soil)
Free ammonia or macros with no uptake.
Quick test: Bloom after a big feed or EI dump.
No UV / no blackout, only water changes
You dilute cells; they rebound with light.
Quick test: 50% WC daily, still green next afternoon.
Confusing it with cyano mats
Cyano sits on sand; green water is the water itself.
Quick test: Substrate clean, water green.
Fix order
-
Step 1
Confirm green water, not glass film
Flashlight + wipe test. Glass film → GDA page. Grey milk → cloudy-water page.
-
Step 2
Blackout 2–3 days OR UV
Cover completely; keep filter and air running. Alternatively run a UV sterilizer 24–48h. Don’t feed.
-
Step 3
Resume short lights + plant mass
6–7h only. Add cheap fast stems. Pause heavy dosing until water stays clear a week.
-
Step 4
Modest water change after clear
20–30% once the tint fades—not daily 80% during the bloom unless ammonia demands it.
When to escalate
- Green water + ammonia with fish → water changes + reduce feed; blackout still OK with aeration.
- Slimy sand mats → cyanobacteria, not UV-first.
- Calculators → all tools.
Helpful products
Related guides
- Cloudy bacterial bloom — Grey vs green
- Green dust on glass — Film vs soup
- Cyanobacteria — Mats
- Algae hub — Parent
FAQ
Is green water toxic?
Usually cosmetic. Test ammonia/nitrite—those kill, not the green tint.
Will a filter floss clear it?
Cells are too small. UV or blackout, not floss.
Same as cyanobacteria?
No. Cyano is a mat on surfaces; green water is suspended cells.
How long should UV run?
Often 24–72h until clear, then you can pulse. Fix light hours or it returns.
Can I keep feeding during blackout?
Skip or tiny pinches. Extra food feeds the bloom.
Do water changes alone fix it?
They dilute briefly. Without light cut or UV, it usually rebounds.
Aqua Plants Hub Editorial
