You can always count on a fish to be reliable and skilled at two things -- swimming and eating. If you notice your fish seems to have lost its appetite, something's awry. Sometimes a fish's reasons for not eating are simple and straightforward, but not eating can also indicate a more serious issue. By observing both your feeding and your fish's eating habits, you can hone in on the potential problem.
The most common reason for a fish not eating is overfeeding by the owner. With good intentions, an owner will sometimes provide a fish too much food, assuming it's hungry and either wants or needs a greater amount of food than is necessary. The general rule of thumb for feeding is to give the fish the amount of food that it's capable of consuming within two minutes. Note the amount the fish eats, then begin giving just that amount. Cleaning up any excess or uneaten food from the aquarium, too, will help keep the fish's environment clean and in good condition -- something that can also impact a fish's appetite.
Like people, a fish can suddenly turn finicky towards a regular or new diet. Your fish, for example, might eat a particular brand of fish flakes just fine for many months, but then decide it doesn't want those flakes anymore. On the other hand, if you have changed your fish's diet -- having chosen a new kind of flake or pellet -- your fish might be rejecting its new food. Try a range of foods on your fish to see what its favorites and least favorites are. Then, either stick to a diet that a pedant fish likes, or try varying the diet to give an indecisive fish a variety of options.
All fish are sensitive to changes in their environment and can respond by not eating. If you have just recently brought your fish home, it might simply need to acclimate to its surroundings before feeling comfortable enough to eat. If you've owned your fish for some time, look at factors like water temperature. A fish relies, in part, on water temperature to metabolize food. Water that's too warm or too cool can affect your fish's metabolism, energy and appetite. You should also test your water's ammonia and nitrate levels and make any necessary changes to bring the water's quality up to par. The specific needs of your fish's water temperature and nutrient levels will depend on its species.
Once you have ruled out overfeeding, dietary preferences and environment as reasons for your fish not eating, check the fish for signs of illness or disease. Observe your fish for color loss, lethargy, clamped fins, bloating, swelling, open sores, ripped or bloody fins, parasites, fungus or any other unusual symptoms. Closely monitor your fish's eating habits, including how often you're attempting to feed the fish, how much and of what. If, after three days, your fish still hasn't eaten anything, take it to a veterinary clinic to have a professional evaluate your fish for an illness or disease. A veterinarian will be able to more accurately diagnose and treat your finny pal.