Knowing your fish – how they eat, how they swim, how they look, and whether they like the surface, middle, or bottom of the tank – will be your greatest help in telling whether they are sick or not.
As with any other living thing, when fish are sick they look and behave differently. Recognizing these changes early can mean the difference between life and death, not only for the affected fish, but for all the fish in your aquarium. In such a small and closed ecosystem as your aquarium, once a disease, fungus or parasite affects one fish, it can quickly spread through the water to all the others.
Signs of Illness
Fortunately, the signs of illness are pretty much the same for all species of fish. They include:
If fish are swimming slowly or with clamped fins, gasping at the surface, listless, not eating, or if their gills are purplish rather than bright pink-red they are not getting enough oxygen. Check your water's pH, nitrates and ammonia levels. Do a few water changes and check your filters.
Bloating and Enlarged Eyes
Perhaps a bacteria infection, perhaps not. This is referred to as "dropsy" and is difficult to treat. Quarantine your fish in a separate tank.
This may be a matter of a fish being nipped at by an aggressive tank mate. Damaged fins, however, can lead to fin rot, which means an infection by bacteria or fungus. It's best to quarantine a fish until its fins heal. If cotton-like patches appear on the fins or white spots, the fish should be treated with an antifungal for the former and an antibacterial for the latter. Check with your veterinarian or fish store for treatment possibilities.
Parasites on Body
Parasitic worms can enter the tank from new fish or new plants. If they attach themselves to your fish they can quickly kill them and move on to another victim in the tank. Quarantine the fish, remove parasites with tweezers and treat spots with a touch of mercurochrome.
White Spots On Fins: Scraping Against Plants or Filters
These are sure signs of "ich," Ichthyopthirius multifiliis, and it's caused by a protozoan parasite. Don't bother with quarantine since if one fish is affected, the creature or its offspring are present in the entire tank. Various ich treatments are available at aquarium shops.
These are the most common diseases that will affect your fish. While fish stores sell a number of antibiotics and antifungal treatments the best defense against these diseases is to maintain the water quality of your tank, do frequent water changes, make sure the new fish you buy are healthy, keep new fish or plants in quarantine for a week or two before putting them into the community tank, and knowing how your fish behave in order to recognize possible problems when they first appear. At the first sign of a problem, quarantine the fish, but don't be too eager to begin treatment with antibiotics or antifungals since these can do as much harm as good if used too frequently.