Fish don't cough, sneeze or plead to stay in bed, but you'll know when they're feeling under the weather. Pay attention to odd swimming or sluggish behavior, both of which may signal illness in fish. Causes
Lethargic behavior may result from water quality problems, nutritional deficiencies and health problems such as parasitic infections. Contrary to popular belief, sick or lethargic fish don't always stay or swim near the tank's bottom. In fact, fish with respiratory conditions often seek the elevated oxygen levels near the water's surface or the aerator, where they will float or avoid swimming to conserve energy.
Sick fish usually park themselves among plants or rocks to conceal their illness from tankmates and would-be predators. Remember, it's in a fish's best interest to appear healthy and alert. In the wild, the weak make easy prey. So if your fish is reluctant to swim or spends more than 24 hours avoiding you and his companions, examine him closely for signs of illness. Spots, red skin and fins, weight loss, abdominal enlargement and inflamed gills all indicate problems. What You Can Do at Home
First, test the water for ammonia, nitrite and pH imbalances. Also check the thermometer to make sure the heater is working.
Whenever you suspect illness, it's a good idea to perform an immediate 25 percent water change. This simple procedure can dilute pollutants or disease-causing organisms within the tank and buy you valuable time to solve your fish's problem. Always dechlorinate the fresh water and match its temperature and pH to the tank water. When to Call Your Veterinarian
If your water analysis doesn't provide an answer, consult a qualified fish veterinarian right away. The doctor will examine your pet, recheck your water parameters and obtain a thorough medical history. If you can't find a fish veterinarian in your area, consult local pet store employees about your fish's behavior.
One of the things we love most about our fish is their robust energy. If your swimmer seems a little down in the dumps, immediate care may restore his usual vim and vigor.