Algae guide
Blue-Green Algae in Fish Tanks: Causes, Risks, and How to Treat It
Last updated: May 2026
- What it is: Photosynthetic cyanobacteria masquerading as algae—thin blue-green blankets with an earthy odour peeling off décor in sheets.
- Severity: 4 / 4 — Serious — can release cyanotoxins and choke surfaces fast
- Primary cause: Low nitrate arenas combined with stagnant flow and uneven lighting paradoxically worsen some strains; phosphate-limited rigs also report stubborn mats.
- Primary fix: Blackout aquarium 5–7 days, optimise flow, selectively siphon slime, escalate to erythromycin only if biodiversity impact accepted.
Recognition checklist
Mat sheen that lifts like wet tissue paper under gentle flow, rotten smell on disturbance, rebound within days after clumsy brute-force cleaning.
Why it threatens fish health
Some planktonic strains release hepato- or neurotoxins stressing invertebrates first; mats smother moss and bottom dwellers shelters.
Stepwise treatment
Mechanical: Gentle daily siphon of liftable slime without shredding into water column turbulence.
Photoperiod / blackout: Full blackout starves filamentous cyanobacter strands—pair with boosted aeration for oxygen reassurance.
Antibiotics: Erythromycin nukes offending bacteria colonies but collateral hits aerobic biofilter guardians—budget for mature media rescue or ammonium watchers.
Wise husbandry tweaks: Stop chasing zero nitrate blindly; widen testing window versus feeding and plant uptake.
If stock crash signs appear, escalate as you would sudden losses—see our sudden losses guide and sequential mortalities playbooks.
Prevention roadmap
Random-flow gyres, purposeful detritus export, restrained ultra-pure rinsing regimes and calibrated plant feeding keep matrices less inviting.
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