The process of selecting an individual invertebrate or fish should begin with firsthand observation of the animal for as long a period as is possible. It is very important that you observe their feeding habits, especially of those with which you are unfamiliar. Of course this is not always possible within the confines of a pet store or dealer's facility, and certain individuals may be newly acquired and not yet well settled (in which case you should refrain from purchasing them until they have had a chance to acclimate to their new surroundings). It should, however, be taken as a bad sign if the seller refuses to allow your reasonable requests in this regard. It is particularly important that you assure yourself that delicate creatures, especially marine or freshwater fish that consume live food only, are well acclimated to captivity and feeding prior to purchasing them. Consider placing a deposit on a specimen of particular interest to you and returning at later dates to observe its progress if you are not satisfied at your first visit.

When choosing animals for your aquarium, bear in mind that behaviors that indicate a problem in one species may be normal in another. For example, clamped fins, a sign of poor health in most freshwater fish, are usual for many marine species. Most eels breathe by rapidly gulping water, a behavior that would indicate distress in many other types of fish.

Animals that you select should show evidence that they are in normal body weight range. This is nearly impossible concerning invertebrates, due to their hard exoskeletons, but with some practice, you can become adept at determining whether or not most fish have been feeding. Even species that are normally elongated or thin in body shape, such as loaches and eels, will display pinched stomachs if they are in poor condition. Fish that are encased in bony coverings, such as seahorses and cowfish, will nonetheless exhibit an indented stomach if they have not been feeding regularly.

It is important that you understand and recognize the normal behavior of the species that interests you, because typical behaviors can provide clues as to the animal's overall health. For example, flounders generally do not swim about much, and their doing so may indicate a problem. Welladjusted individuals will be found partially buried in sandy substrates or just above it. Sessile invertebrates should be firmly anchored to a substrate and the shells, where present, and should not be gaping widely or loosely attached to each other. There are innumerable considerations to be aware of, and those concerning each species are different. The point is that you must go shopping well armed with a detailed understanding of and knowledge about the creatures that you intend to purchase.

You should know, for example, that activity is not necessarily a sign of good health. Many fish become agitated when stressed by disease, parasites, or the lack of oxygen. Invertebrates such as crabs and crayfish should, in general, be hiding unless they are foraging for food. Those scuttling madly about the aquarium, outside of feeding time, are not exhibiting normal behavior.

The eyes of most fish should be clear and the animals should be alert within the confines of their normal behavior. Search carefully for signs of fungus, which may appear as dark or light films on the body, and for parasitic infections in the form of small swellings or discolored areas on the scales. Of course, you should take note of obvious injuries such as wounds and frayed or missing fins. Most fish can regrow torn fins, and invertebrates often replace lost legs at their next shedding, but such may indicate that the animals have been improperly transported or housed by the seller.

Avoid fish that are constantly rubbing up against substrates or rocks, because this may indicate that they are attempting to remove external parasites from their scales. You should be completely familiar with the normal coloration of the fish or invertebrates that you plan to purchase. Individual and regional variation will come into play, but avoid those that are dull or lackluster in appearance (unless, of course, this is their normal coloration). Be aware also of the species' usual rate of respiration, as bad water quality or other stress factors may cause fish to respire at an abnormally high rate.

The overall health and condition of invertebrates is much more difficult to assess than that of fish. The rigid exoskeletons of most give little clue as to whether or not they are in good weight, and their movements may not be of much diagnostic value either. Sessile invertebrates are particularly difficult to evaluate, but you should check that they are attached to a substrate, and that any visible tissues are intact and free of fungus or unusual marks. Soft-bodied invertebrates such as sponges and sea cucumbers should be checked for wounds or other injuries and for discolored areas on their bodies. The spines of sea urchins and missing legs of crayfish, lobsters, and sea stars will regrow, but the process places an additional strain on the animal's bodily functions.

Be especially aware of the condition of the aquariums in which the animals that you intend to purchase are kept, and of the state of repair and cleanliness of the store or dealer's facilities in general. Of course, animals offered for sale in commercial establishments cannot be housed in complicated, well-decorated aquariums due to the constraints imposed by largescale operations. However, the animals' basic requirements should be met. Here, again, you will need to know a good deal about the animals themselves if you are to be able to make an accurate assessment of commercial facilities. Although the tanks in most pet stores and dealers will, of necessity, be fairly barren in terms of decorations and plants, aquatic organisms that are particularly delicate or have very specific needs should be housed with such considerations in mind. Moray eels, for example, will be extremely stressed unless they are able to hide in a secure retreat. Housing such creatures in a bare aquarium will almost guarantee that they become ill from a stress-related disorder. A seller who lacks a basic understanding of or concern for his charges is likely to make mistakes that jeopardize the overall health of the stock.

Especially for animals that are difficult to acclimate to captivity, you should request details as to how the animal has been maintained and what it has been eating. Particularly important are parameters such as temperature and pH and, for marine animals, specific gravity.

For animals of special concern, especially those that are rare in captivity and/or in nature, you should also attempt to obtain background information as to the place of origin. Species with large natural ranges may experience extreme differences in climate, behaviors, and food preferences over the expanse of that range. You will need to adjust for such factors if you are to successfully meet the husbandry challenges that such creatures present. This is especially important if you are concerned, as you should be when dealing with such animals, with captive reproduction. Many invertebrates and fish are stimulated to breed by fluctuations in day length and temperature. Animals originating in the more northerly parts of their ranges will experience significantly different seasonal fluctuations than will those from the south.

You should make every effort to determine the origin of any particularly rare animal that you maintain. If at all possible, the breeding of such creatures should occur only with other individuals from the same home range, so as to preserve any genetic differences that may exist between local populations of the same species.