Although some diseases of aquarium animals are indicated by very specific and easily recognizable symptoms, in many cases you will need to rely upon generalized changes in behavior or condition when attempting to diagnose an illness. As always, you will need to understand your pets' normal behaviors to be able to quickly recognize any deviation from them.
Even allowing for the vast differences in the behavior of the many species of fish and invertebrates, certain general patterns may indicate to you that a closer look is necessary. Animals that swim about without normal pauses or sedentary creatures that suddenly begin to wander about rapidly may be indicating that the water quality is poor or that they are under pressure from an aggressive tank mate. At the other end of the spectrum, sluggishness or the adoption of an unusual posture may also signal that something is amiss. Seahorses, for example, usually remain anchored to coral or plants, even when feeding. Seahorses that spend a good deal of time swimming about the aquarium, Even though this is behavior that is quite normal for many other fish), are definitely under some type of stress.
Is it safe to medicate fish or invertebrates if I cannot determine the exact nature of their illness?
Broad-spectrum medications that treat a variety of commonly encountered diseases are available for marine and freshwater aquariums. Be sure, though, that a disease and not an environmental condition is involved. If the distress is caused by a factor such as improper water temperature, medications will do more harm than good.
Take note of the usual respiration rates of the fish that you keep. Any increase in this rate, especially if the fish gulps air at the surface of the tank, may indicate that the oxygen content of the water is too low. A number of factors can cause a drop in oxygen levels, including overcrowding, a filter that is malfunctioning or too small, or a sudden spike in temperature (water's ability to hold oxygen decreases as the temperature rises). Similarly, you should be familiar with each of your pets' modes of feeding and the usual states of their appetites. In most cases, a reluctance to feed indicates illness or, in the case of females, it can indicate that she is developing eggs or young.
Although the diagnosis and treatment of the diseases of fish and invertebrates has come a long way in recent years, knowledge in this area is still far from complete. Therefore, prevention of disease will be a far more effective tactic than looking for a cure. Fortunately, by careful research and discussion with others, you will likely be able to avoid many of the pitfalls that befall those who do not take the time to learn about their pets.
Be sure to take careful notes concerning your experiences, and do not be afraid to engage in “educated experimentation” when it comes to diagnosing and treating disease. Especially for those conditions that people are currently unable to cure, you would do well to apply your interest and knowledge to serious experimentation. Above all, remember to share the results of your work.