The physical condition of each of your pets should be checked every day. The most convenient time to do this is generally during the morning feeding, because most of the animals will be hungry and therefore active at this time. Your most important ally in detecting illness and disease among aquatic animals is your own familiarity with their normal behavior, feeding habits, and positions within the aquarium. All wild animals, fish and invertebrates included, possess an uncanny ability to hide signs of sickness and injury. In the wild, this strategy serves a very useful purpose in that predators almost always focus on animals that stand out by appearing weaker and thus less able to escape or fight back. In captivity, however, the propensity of animals to hide their discomfort complicates the work of the hobbyist in evaluating their health. Any unfamiliar behavior or posture should be thoroughly investigated.

When you observe your pets as they feed, be sure to check that all are getting a sufficient portion of the food, as dominant animals will inhibit others from feeding properly. This behavior is not always readily apparent, especially during the frenzied feeding activities of a good number of fish in a large aquarium. Therefore, you must pay particular attention to each individual. You should also learn to recognize what a healthy individual fish looks like. This varies greatly for each species, but even fish that are naturally thin or elongated in shape will show pinched bellies or other signs that they are not feeding well. Observing healthy animals in public aquariums or reliable pet stores will go a long way toward helping you in this regard.

It is important to remember that signs of illness will be even harder to notice in less active fish and, especially, in sessile invertebrates. Therefore, be sure to check carefully each sponge, clam, coral, and other such creature that inhabits your aquarium. The deaths of such animals are easy to overlook. Decaying corpses that go unnoticed can quickly pollute your aquarium and kill the other animals living there. Water quality can deteriorate rapidly upon the death of an organism within the aquarium, even if the creature is quite small in size. Be sure to perform an ammonia test immediately upon the discovery of any dead fish or invertebrate in your aquarium.