Aquatic Medicine: Water Quality, Osmoregulation, and Aquatic Anesthesia
Core concepts
Aquatic medicine questions reward candidates who understand that water is the patient's environment and its diagnostic sample. For fish, the essential parameters are dissolved oxygen, ammonia, nitrite, nitrate, pH, temperature, salinity, and alkalinity/hardness. Ammonia and nitrite toxicity are the most common causes of acute mortality in captive systems; ammonia toxicity worsens at high pH and high temperature because more is in the un-ionized (toxic) form.
Understand osmoregulation: freshwater fish are hyperosmotic to their environment and actively take up ions and excrete dilute urine; marine teleosts are hypoosmotic and drink water while excreting concentrated ions via gills and kidney. Elasmobranchs retain urea and TMAO to match seawater osmolality. This explains why freshwater fish tolerate poor ion uptake differently than marine fish and why rapid salinity shifts are dangerous.
Know aquatic anesthesia and restraint: MS-222 (tricaine) is buffered to avoid pH drops; eugenol/clove oil is used but has welfare considerations; benzocaine is poorly water-soluble. Support anesthesia with recirculating water over the gills.
Traps
Questions often present a fish with normal ammonia but high nitrite - nitrite causes methemoglobinemia (brown blood) and is treated with chloride (salt). Another trap is confusing pH with alkalinity. Also, do not assume a marine mammal question is about fish water quality - marine mammals need terrestrial-style hematology and respiratory monitoring.
Worked example / rule of thumb
Rule of thumb: "Ammonia, nitrite, oxygen, pH, temperature - check all five before treating any fish." If a system shows ammonia > 0.5 mg/L with high pH, do an immediate water change and reduce feeding; do not simply add a commercial bacterial starter and wait.
Exam strategy
When the stem gives water quality values, the answer is usually environmental correction first, then supportive care (oxygenation, salinity adjustment, chloride for nitrite).