For fish to breed successfully in the aquarium, they must be healthy and feel that all conditions are right. Generally, it is difficult to breed fish caught in the wild, as they are much more demanding in their requirements, but usually even the most awkward types can be bred by expert aquarists. After the first few generations have been bred, further breeding is progressively less difficult, as the 'easy breeders' become selected out from the population.

Thus fish which, when first imported, commanded very high prices become quite cheap once the principles of breeding have been mastered. For example, the Neon and the Cardinal were originally very expensive and bred only and with great difficulty by the specialist, but now both are standard aquarium fishes, available everywhere at moderate prices, and easily bred.

Among the species which cannot be bred easily are those which become very large, and in which only the immature fish are kept in the aquarium. Some African Catfish fall into this category, as do many marine fish. Usually, however, breeding can be accomplished once the environment and biology of the fish are completely understood.

One curious factor in breeding is that, although the breeder can often select out natural mutations with unusual colors or long trailing fins, and develop new strains, sometimes the general quality deteriorates in captivity. For example, it is seldom possible to preserve the huge dorsal fin on the Sail fin molly or the bright colors of the Cichlid Apistogramma ramirezi when these fish are bred in captivity. Wild stock are larger, more robust and more brightly colored.

Breeding considerations have given rise to much of the interesting behavior of fish. Even coloring can be traced back to breeding needs. Often male and female fish are colored quite differently, and male fish may be brightly colored both to attract females and to frighten off rival males.

Most fish do not breed continually. The experienced eye can pick out fish which are pairing off as they reach peak condition for breeding. The sides of the fish fill out, and females, packed with eggs, become especially plump. Coloring becomes generally more intense, and males may display to the females with rigidly out-stretched fins. When spawning is imminent, there is often a change in the fishes' behavior: they tend to become very active and to explore possible spawning sites.

The sexual organs of fish are relatively simple, consisting of paired, elongated organs lying along the abdominal cavity these organs produce either eggs or sperm (milt). Nearly all fish shed both eggs and milt together into the water, where fertilization takes place. However, a few ,are live-bearers; that is, the male introduces his milt into the female, who retains the fertilized eggs and gives birth to live young. The Guppy is the best known of these fish.

Fish eggs are thin-walled and delicate, and are eaten greedily by most species, many consuming even their own spawn. Various special techniques have been evolved to ensure the survival of fishes' eggs or, at least, of enough of them to ensure the continuation of the species. Indeed, the method evolved by most fishes is to produce such huge quantities of eggs that some are bound to survive.

These egg scatterers are usually shoaling fish, which spawn in a group. Our common river fish are of this type, and so are their exotic relatives which are kept in the aquarium. Fishes like Barbs, Danios and Rasboras all scatter their eggs among plants. Sometimes the eggs are sticky, and become attached to the plants; others simply drop to the bottom.