First, it is important that your fish’s environment is a stable and healthy one. Water temperature, PH, and other factors can have effects on your fish’s appetite(s).
It is also important to ensure that you are feeding your fish the correct type and amount of food for the particular breed. This means doing your homework, and knowing all you can about the breed(s) of fish you own.
Fish may not eat for more serious reasons, such as injury or bacterial infections. Since it is impossible to accurately diagnose bacterial disease in fish merely by sight, it is important that other tests be performed on the fish, such as mucus sample testing. Injury to your fish will be more obvious. Either way, early diagnosis and treatment is extremely important, as these can be fatal to your fish.
There are many sources, such as books and charts, which can help you diagnose fish disease or injury. Local fish supply stores, employees, and other fish owners are also a great resource.
Whether your fish will not eat because of improper environment or because of injury or disease, it is important to pinpoint the problem. This will allow you to correct the problem or treat the injury or disease, in order for your fish to have a happy, healthy life.