Table: Common Environmental Diseases of Fish - Merck Veterinary Manual

Common Environmental Diseases of Fish

Common Environmental Diseases of Fish

Disease	Problem	Behavioral Signs	Physical Signs	Acute Mortality	Chronic Mortality
Low dissolved oxygen	Hypoxia	Piping at surface; large fish affected more than small ones	Flared gills, dark	Catastrophic mortality	Ongoing
Gas bubble disease	Supersaturation (often N₂ gas)	Lethargy, buoyancy problems	Gas bubbles in gill capillaries, fins, and eyes; exophthalmos (“popeye”)	Catastrophic mortality	Ongoing
Carbon dioxide toxicity	CO₂ >40 mg/L	Lethargy at surface	None	Yes	Yes
Ammonia toxicity (new tank syndrome)	Un-ionized ammonia >1 mg/L pH >8	Lethargy, anorexia, spinning, convulsive swimming	Darkened	Catastrophic mortality	Ongoing
Nitrite^a toxicity	NO₂ >0.1 mg/L^a Low chloride (freshwater)	Piping at surface	Gills and blood dark brown color	Yes	Yes
Nitrate^b toxicity	NO₃ –N ≥70 mg/L Low (no) iodine, O₃	Lethargy	Swelling around throat ("goiter")	No	Yes
Old tank syndrome	Total alkalinity (TA) ~0 mg/L pH <6, high total ammonia nitrogen (TAN)	Lethargy, poor appetite	None	No	Yes
Lack of minerals	Total hardness <20 mg/L Use of distilled water	Sudden death	None	No	Yes
Chlorine toxicity	Detectable chlorine	Acute: sudden death Chronic: lethargy, irritation	Acute: none Chronic: excess mucus, cloudy eyes, gill inflammation/ necrosis	Catastrophic mortality	Ongoing
Hydrogen sulfide toxicity	Detectable H₂S (rotten egg smell)	Acute: sudden death Chronic: lethargy, poor appetite/growth	None	Catastrophic mortality	Ongoing
Copper toxicity	Cu²⁺ >0.2 mg/L Low alkalinity	Sudden death	None	Catastrophic mortality	Ongoing
Zinc toxicity	Coins in exhibit Stainless steel, acidic pH	Lethargy, anorexia	None Enlarged abdomen (coins in GI tract)	Not usually	Yes
Stray voltage	Electrical short, mechanical problem	Irritation, mortality	Broken back	Yes	Yes
^a There is tremendous species-specific variation to nitrite toxicity. Environmental conditions, especially chloride concentration, can mitigate the effects. Most test kits measure mg/L NO₂−N. To convert to NO₂ (mg/L) concentration, this value must be multiplied by 3.3. ^b Most test kits measure mg/L NO₃−N. To convert to NO₃ (mg/L) concentration, this value must be multiplied by 4.4.