Marine bacterial infections present as ulcers, fin rot, mouth rot, or generalized lethargy. Most commonly Vibrio, Aeromonas, or Pseudomonas species. Stress + parameter instability + injury are the predisposing factors.
See /care/diseases/fin-rot/, /care/diseases/columnaris/, /care/diseases/mouth-fungus/ for freshwater-specific protocols.
Move affected fish to hospital tank. Dose Kanaplex (kanamycin) + Furan-2 (nitrofurazone) combo for 10-14 days. Daily 25% water changes during treatment. Lower salinity slightly (1.022) reduces osmotic stress. Add Beta-glucan or Garlic Guard to support immunity. Address underlying cause - check parameters, look for aggression injuries, test for elevated ammonia or nitrite.
Quarantine new fish 4-6 weeks with prophylactic copper or formalin. Maintain low nitrate (<10 ppm reef, <25 ppm FOWLR). Avoid mixing aggressive species. Stable salinity (refractometer-checked weekly). Pre-mature reef rocks 4 weeks before adding fish.
Estimated cost: $50-100.
Bacterial: ulcers, fin rot, white patches, lethargy without visible parasites. Parasitic (ich, velvet, flukes): visible spots, scratching, rapid breathing. When in doubt, treat for both - dose Kanaplex + Furan-2 (bacterial) plus copper or chloroquine phosphate (parasitic) in separate hospital tanks.
Vibrio and Aeromonas are zoonotic but rarely infect humans with intact skin. Wear gloves when handling sick fish or when you have open wounds on your hands. Contact a doctor if you develop a wound that doesn't heal within 7 days after handling sick aquarium fish.
The single most important disease-prevention step: a 4-6 week quarantine of every new fish before adding to your display. See the complete quarantine protocol.