Fish Not Eating: Causes of Appetite Loss and What to Check

A fish that suddenly stops eating is often reacting to water quality, stress or infection. Learn the common causes of appetite loss - ammonia and nitrite, new-tank syndrome, parasites and low temperature - the red flags, and how to respond.

Why a fish stops eating

Appetite loss is one of the earliest and most common signs of trouble in aquarium fish. Because so many problems suppress feeding, it is best read together with water tests, recent changes in the tank, and any other symptoms.

Common causes

• Water quality. Ammonia and nitrite exposure causes lethargy, anorexia and darkened color.
• Stress or a new environment. In new tanks, an ammonia and nitrite spike ('new tank syndrome') leaves fish lethargic and anorectic during the first weeks and can be fatal.
• External parasites. Loss of appetite is among the early signs of ich and velvet.
• Internal parasites such as Hexamita and Spironucleus, which cause inappetence with weight loss and emaciation.
• Bacterial infection, as part of a systemic illness.
• Temperature too low for the species, which depresses feeding and immunity.

What to check

Test ammonia, nitrite, pH and temperature. Review recent changes - a new tank, new fish or transport. Look for parasite spots or flashing, abnormal (white, stringy) feces, and body lesions or a pinched, hollow belly.

Approach

Test and correct water quality and stop overfeeding first - reduce feeding and do water changes for new-tank and ammonia cases. If parasites or emaciation are present, pursue the specific diagnosis and treat it (for example, metronidazole for internal flagellates). A healthy fish that briefly refuses a new food is different from a fish that is unwell; judge by the whole picture.

Sources: Merck Veterinary Manual, Environmental Diseases of Aquatic Animals (www.merckvetmanual.com); Merck Veterinary Manual, Management of Aquarium Fish (www.merckvetmanual.com); Merck Veterinary Manual, Parasitic Diseases of Fish (www.merckvetmanual.com); UF/IFAS FA006 (ask.ifas.ufl.edu); The Fish Site, Parasitic Diseases of Tropical and Ornamental Fish (thefishsite.com).