In most of the fish that we have in the aquarium you can tell that it is sick simply by the vision and behavior of the same. Although later there are obvious signs, by which the disease is identified. When we talk about a sick fish it is because we have noticed that it has modified its usual behavior, and from there, we move on to identify the disease to restore the fish's health.
The coloration, the way of swimming, the voracity or absence of it, withdrawal, folded fins, isolation of the shoal, irregular swimming, determine, is abnormal and therefore we have to remedy, according to the fish species.
There are abnormal behaviors that indicate that the fish is sick and that they are common in all fish. Rejection of the usual food, folded fins, irregular swimming or isolation in the corners of the aquarium, back and forth movement, rubbing against stones, objects or aquarium floor, agitated breathing and lack of reaction when we try to catch them with a net.
When a fish changes colorIf the change is circumstantial we should not worry, but if it persists we would be talking about a disease and observing the fish to detect it. We could be talking about anemia, since it produces discoloration in the fish, either due to the lack of oxygen in the aquarium or the poor lighting of its system or a parasite that has settled on the skin.
If the fish has the sunken bellyWe would be talking about malnutrition, rickets and tuberculosis. A bulging belly It could be a constipation of the intestine, ascites, or dropsy. The last two diseases are serious, since they are produced by attacks of bacteria, sometimes associated with myxobacteria, which is highly contagious and difficult to cure.
It is important to note that in the event of any anomaly that we see in the fish we must isolate it from the rest of the fish until you know your diagnosis and treat the disease.