There are common tell-tale signs to help in diagnosing if your fish is ill and they are as follows: clamped fins, frayed fins, bloating, enlargement of eyes, erratic swimming, gasping for air at the surface of the water, scraping on plants and filters, loss of appetite, white spots on fins/body, parasites on body, and normally active fish who remain still. Conditions or symptoms may vary.
There are known visible signs that will indicate if a fish is dying but, according to PetPlace.com, the most helpful tool is knowing how they eat, how they swim and where their favorite area of the tank is located. Knowing these things can help identify what has gone wrong with a fish. Being familiar with the characteristics and behaviors of a fish makes it easier to identify when the fish changes its behavior patterns. Most of these symptoms apply to all kinds of fish and can easily be used to discover what is making a fish sick.
The advised course of action when deciding if a fish is dying is to observe it for about twenty minutes. After discovering the symptoms, it is recommended to check online resources to find the diagnosis of a fish. If the problem cannot be easily identified, contact a veterinarian for a professional opinion.