Hybrids can be produced due to many different reasons, and hybridization does occur in the wild as well. In captivity, one common reason for hybridization is the aquarist who tosses down closely related species into the same aquarium without caring about the risk of hybridization. Hybridization can also happen involuntarily as it can be really difficult to distinguish certain species from each other. Especially drab colored females from various closely related species can be virtually impossible to tell apart from each other.
There are also those who willingly produce hybrids since they want to create more appealing fish for the aquarium market, such as fish with striking colors, and patterns or intricate finnage. Hybrids can also be when an aquarist really, really wants to breed his or her fish but fails to find a suitable mate from the same species. In such situation, some aquarists are tempted to try their luck with a closely related species instead.
A lot of fish species are today under threat of extinction in the wild and keeping them alive in aquariums is one way of preserving them for the future. Against this background, it is easy to see why hybridization can be a problem. The situation becomes even more problematic when aquarists sell their hybrids as non-hybrids. Some people erroneously believe that all hybrids are sterile, but this is very far from the truth, especially in fish.
Another problem has to do with rules and regulations regarding collection and import of live fish. When a species becomes endangered in the wild, it is common to ban harvest and marketing of that species in order to prevent the few remaining specimens from being removed from their natural habitat. Once a ban is enforced, the aquarium hobby has to make do with specimens that have already entered the hobby. Using these specimens for hybridization is naturally not desirable.
If we want to preserve the distinctness of each population, we must refrain from mixing fish from different populations in the aquarium. It is therefore recommended to work within each population and only breed fish hailing from the same area.
You can avoid hybridization by keeping closely related species in the aquarium away from each other. Do not mix distinct populations of the same species. Try to find out the ancestry of your fish before you start breeding them. Be suspicious when new species of fish suddenly enter the market, they can be hybrids that someone is trying to pass of as a new species.