You walk by your aquarium, and something catches your eye. You stop and turn to take a closer look - something doesn't look right. Upon closer examination, you see that a fish is not behaving normally, or there are external signs of disease. If you have been keeping fish for any length of time, you have experienced a similar scenario. Common signs of possible disease that an aquarist might bump into are as follows:
Altered or decreased activity
These are common signs of disease, but they are not specific to one disease and may be seen with poor water quality, various infectious diseases and many other problems.
This is a nonspecific sign that can be seen with a variety of diseases.
Flashing or glancing. The act of fish rubbing up against objects in the aquarium may be a sign of irritated skin. This is commonly observed with fish afflicted with external parasites.
With fish that normally school, an individual isolating itself from the group can be an early indication of disease.
Respiration concerns. Gasping for air at the surface is a sign of either poor water quality (including low dissolved oxygen) or gill disease (parasites, bacteria, viruses). Also look for increased breathing rates and crowding near the filter return at the top of the water column.
Skin lesions are one of the most common signs of disease and are often the first problem observed by hobbyists.