Merck Manual

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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

Nitritea toxicity

NO2 >0.1 mg/La

Low chloride (freshwater)

Piping at surface

Gills and blood dark brown color

Yes

Yes

Nitrateb 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.