Fish Practice
Core concepts
Fish medicine is dominated by husbandry and water quality. The candidate must understand the nitrogen cycle: ammonia and nitrite are toxic, and biofiltration converts them to less toxic nitrate. Know the parameters that drive fish health - temperature, pH, dissolved oxygen, hardness, and salinity - and recognize that most fish disease is environmental in origin. Familiarize yourself with the anatomy that differs from mammals: gills for gas exchange and nitrogenous waste, a lateral line, and a swim bladder.
Diagnosis and treatment
Diagnostics include gill and skin wet mounts, water chemistry testing, and necropsy. Treatments must be delivered through water or feed, and drug dosing is species- and system-dependent; many antibiotics are restricted in food fish and in systems with biofilters. Quarantine is the cornerstone of prevention.
Traps
Questions frequently test whether you investigate the environment before treating the fish. Another trap is assuming that a medication safe in a display tank is safe in a system with invertebrates or that it will not harm the biofilter. Watch for questions about regulatory limits in food fish.
Worked example
A hobbyist reports multiple fish gasping at the surface with red gills. Rather than reaching for an antibiotic, the first step is to test ammonia, nitrite, and dissolved oxygen and review stocking and filtration. Rule of thumb: in fish, the tank is the patient - treat the water first and the fish second.