Recognition of the cause of a fish disease is the condition for successful treatment. Basically, we have to differentiate between diseases caused by pathogens and diseases whose causes reside in the fish; hereditary diseases, deformities, toxicity, injuries, and improper diet. The recognition and control, of these causes are described on another article on this site. Diseases caused by pathogens are more frequent. Pathogens are all living organisms that can cause disease.

If the fish act abnormally, the first step is to find out if all fish or only a few fish act this way. If the symptoms appear within a brief period in all the fish, or if at least the more sensitive species exhibit the same abnormal behavior, poisoning is highly probable. This can be expressed in the most varied ways. During and after the poisoning, you may observe disrupted equilibrium, paralysis, twitching, cramps, darting about the tank at the slightest stimulus and then bumping into things, rapid breathing, gasping for air just under the surface of the water, fading of coloration, discoloration of the gills and fins, red spots on the body, whitish cloudiness of the skin, or increased secretion of mucus.

Depending upon the kind of toxin involved, removal of the cause may bring improvement. If a powerful toxin has affected the fish, they can still succumb later, even in toxin-free water, from the delayed effects of the toxin. Recognition of poisoning is very difficult when only slight traces of the toxin remain in the water. Damage then occurs only over the course of time. It has been observed, that fish died of organ damage only at a mature age, while young fish were not affected.

Diseases caused primarily by pathogens rarely spread quickly to the whole population in well maintained aquariums. It is always just a few fish that are affected first, unless the whole tank has been newly filled with fish that are already diseased. The observant aquarium hobbyist will recognize the change in behavior of a few fish, so there is still enough time to take countermeasures. In order to recognize any change in the fish's behavior, the aquarium keeper must be very familiar with the fish's lifestyle in a healthy state. Behavioral changes that reveal diseases are not expressed in the same way in all species.

Likewise, we cannot conclude from a change in behavior that the fish has a specific disease, for a certain change in behavior may have one or more of many different causes. Staggering along, paler or darkened coloration, and clamped together fins are symptoms of exhaustion and extreme malaise possibly associated with internal damage. Likewise, extremely high or low pH values, heat, cold, and lack of oxygen could be the causes. Swimming on the side, disequilibrium, "standing on the head," and turning over suggest severe damage caused by an infection of the brain, air bladder, or labyrinth (organ of balance), or they can indicate the terminal stage of a disease.

If the fish rub against decorations and plants and twitch their fins vigorously or fold them together, then ectoparasites are affecting them. Spreading of the opercula, associated with rubbing and frequent, rapid protrusion of the mouth, indicates gill worms. Rapid breathing, however, is not a definite symptom of gill worms. The cause can also be a toxin in the water, lack of oxygen, improper pH, or another stressful factor. (See fish disease charts.)