Fin rot is a bacterial infection that attacks fin edges, producing ragged, frayed, or receding fins. Almost always caused by poor water quality or stress weakening the fish's immune response.
Causative organism: Bacterial (typically Aeromonas, Pseudomonas)
Severity: Moderate (treatable if caught early)
Test ammonia, nitrite, nitrate. Do a 50% water change. Most fin rot resolves with water quality alone if caught early.
Sometimes "fin rot" is actually fin nipping by aggressive tank mates. Watch the tank for 30 minutes - rule out behavioral cause before treating bacterial.
For mild cases. Melafix is plant-extract based and gentle. ParaGuard treats broader-spectrum bacterial issues.
Kanamycin (Kanaplex) or erythromycin (Maracyn) for established gram-negative bacterial infections. Treat for 5-7 days. Remove carbon during treatment.
Antibiotics damage beneficial bacteria in display tanks. Move infected fish to a hospital tank for antibiotic treatment if practical.
Antibiotics kill nitrifying bacteria. After treatment, dose Dr. Tim's One and Only or Fritz Turbo Start to re-establish biological filtration.
Fin Rot is preventable in 95%+ of cases by running a 4-6 week quarantine on every new fish before introduction. Read the quarantine protocol.
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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