First assessment
Test ammonia, nitrite, nitrate, temperature, pH and oxygenation and consider recent introductions before choosing medication. Isolate suspicious fish and consult a fish veterinarian when feasible.
Common conditions
White spot disease (ich), ectoparasites including Gyrodactylus, bacterial fin/skin/gill disease including columnaris, velvet, and chronic wasting syndromes can have overlapping outward signs. Microscopy, clinical history and appropriate diagnostics distinguish look-alikes.
Infection control
Quarantine new stock for at least 30 days where practicable; separate tools, use dedicated nets and document losses. Never blindly combine formalin, copper, antibiotics or salt. Copper is hazardous to many aquarium invertebrates.