Cyanobacteria (cyano) appear as red, brown, or black slime sheets that coat substrate, rocks, and equipment. Not technically algae - it is a photosynthetic bacterium. Releases trapped phosphates when disturbed, fueling further blooms.
Excess phosphate (>0.1 ppm) combined with low flow + new tank or post-cycle nutrient imbalance.
Always treat in a separate quarantine tank.
Maintain phosphate <0.05 ppm + nitrate 5-10 ppm. Avoid stagnant low-flow zones. Don't overfeed.
Red, brown, or dark green slime sheets. Bubbles forming under the slime. Sudden appearance after a parameter change.
Excess phosphate (>0.1 ppm) combined with low flow + new tank or post-cycle nutrient imbalance.
Increase flow: Add a powerhead or angle existing flow to disrupt the affected area. Cyano cannot grow in high-turbulence zones.
Maintain phosphate <0.05 ppm + nitrate 5-10 ppm. Avoid stagnant low-flow zones. Don't overfeed.
Mild but persistent if untreated
Browse the full disease database for 45 aquarium conditions with treatment protocols, or check the care library for prevention-focused husbandry guides. Use our symptom matcher to rank likely diseases from observed signs, the water parameter checker to diagnose related water-quality issues, or the QT timeline calculator to plan a treatment schedule.
Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.
Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.
Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.
Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.
Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.
How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.
What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.
How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.
Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.
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