Fish don't diet, so treat slimming as a sign that something's amiss.
Fish come in all shapes and sizes, but if your fish suddenly slims down, it's a clue that he could be sick.
Routinely check out your fish from above, from each side, and from head-on perspectives to gauge body contours. A fish's sides should be gently rounded and not concave. Even a naturally thin-bodied fish should look healthy--not undernourished. If your finned friend starts dropping in size despite regular feedings, it's time to investigate.
Monitor feedings to make sure your slimming fish is actually eating at mealtimes and not just swimming with the crowd. Appetite loss may be the problem, but timidity also can spur weight loss if more aggressive tankmates gobble up all the goodies. You may need to create secluded dining areas with plants and other decorations to make sure the shy types get their fair share.
A fish who eats normally but still loses weight may suffer from a chronic disease. Some fish, especially older ones, suffer a slow decline with such medical concerns. Nitrites, ammonia, and other water pollutants also can cause your pet to lose his appetite and waste away, as can intestinal parasites.
Watch your fish closely for other symptoms of disease and conditions such as skin nodules, colored bumps, open sores, scale sloughing, and lethargy. If you notice any of these signs, act quickly - you may save your pet's life.
What You Can Do at Home?
First, test the tank water for ammonia, nitrite, and pH imbalances. Your local pet store or veterinarian can help if you don't own a test kit. Also check the tank's thermometer to make sure the heater is working correctly.
Next, perform a 25 percent water change. Be sure to dechlorinate the water first and match its temperature and pH to the tank water. This simple process will dilute any pollutant or disease organisms within the tank.
When to Call Your Veterinarian
If the water analysis doesn't provide an answer, consult a qualified fish veterinarian immediately.
If you can't find a fish veterinarian in your area, consult an Associate at your local Petco.