Troubleshoot
Cyanobacteria (Blue-Green Algae) Emergency Fix
Cyanobacteria (“blue-green algae”) is a bacteria mat, not a true algae. Slimy sheets on sand and slow zones, sometimes a swampy smell—treat organics and flow, don’t scrape like hair algae.
- Cyanobacteria
- Blue-green mats
- Organics
- Blackout + flow
What you’re seeing
Look for slimy blue-green or dark teal sheets that peel off sand in one piece. Often a faint pond smell. Hair algae pulls as threads; cyano lifts as a blanket.
| Pattern | Likely driver |
|---|---|
| Mat on sand + low flow corners | Organics + dead flow |
| After overfeed / dirty canister | Dissolved waste spike |
| Low nitrate, high light, still water | Classic planted-tank cyano niche |
| Pea-soup green water instead | Green water bloom—different fix |
Likely causes (ranked)
Low flow + detritus on substrate
Cyano loves still sand with mulm.
Quick test: No surface ripple; mat starts in back corners.
Organics / dirty mechanical media
Overfeeding and clogged sponges fuel mats.
Quick test: Canister not rinsed; leftover food.
Very low nitrate with strong light
Plants stall; cyano occupies the lean bright niche.
Quick test: NO₃ near 0, lights still long.
Treating it like hair algae
Scraping without flow/organics fix brings it back in days.
Quick test: Mat returns within a week after siphon only.
Fix order
-
Step 1
Siphon the mat + increase flow
Hose the sheet off sand. Point outlets across the substrate. Add a small wavemaker if corners stay dead.
-
Step 2
Cut organics hard
Rinse mechanical media in tank water, skip feeding 48h, remove melting leaves.
-
Step 3
Short blackout (2–3 days)
Cover the tank; keep filter running and gas exchange. Then resume 6–7h lights—not the old 10h schedule.
-
Step 4
Rebalance nutrients carefully
If NO₃ was undetectable, modest nitrate dosing after the blackout helps plants—not a dump of macros into still water.
When to escalate
- Mat returns after 2 blackouts + flow fix → consider erythromycin-class treatments only as last resort; they nuke filter bacteria too.
- Livestock gasping → stop cover, aerate, water change.
- Green filaments instead → hair algae.
Helpful products
Related guides
- Green water bloom — If water itself is green
- Hair algae — Filaments vs mats
- Algae ID chart — ID
- Algae hub — Parent
FAQ
Is cyanobacteria dangerous to fish?
Mats themselves are mostly ugly; some strains can stress livestock. Fix flow/organics fast.
Will shrimp eat cyano?
Almost never. Don’t wait on livestock.
Is it the same as green spot algae?
No—GSA is hard dots on glass/leaves. See GSA guide.
Do I need antibiotics?
Try siphon + flow + short blackout first. Antibiotics are last resort and crash cycling.
Why does it smell?
Many cyano mats release a swampy odor—another ID clue vs hair algae.
Can I scrape it off plants?
Lift sheets from sand/hardscape; don’t shred plant leaves. Trim coated leaves.
Aqua Plants Hub Editorial
