Columnaris
Cotton-like white patches on body or mouth. Spreads fast in warm, crowded tanks.
Treatment: Nitrofurazone or Kanaplex for 7 days. Lower temperature to 25°C during treatment.
Velvet
Gold or rust-colored dusting. Especially common in wild-caught and soft-water species.
Treatment: Copper sulfate (not for plants or invertebrates) or acriflavine.
Egg Fungus
Already covered in dedicated article — prevent with methylene blue immediately after spawn.
Fin Rot
Ragged, white-edged fins. Usually bacterial secondary to poor water quality.
Treatment: 50% water change + Melafix or Maracyn for 7 days. Fix root cause (filtration or stocking).
Parasites
Ich (white spots), flukes, worms — quarantine new fish 14 days to catch most.
Preventive dewormers every 6 months for breeding stock.