Many hybrids are fertile and will continue the new strain.
Many hybrids look similar to the original species (especially the females) and these can end up being mixed with the original forms and sold to unsuspecting buyers who then breed them with the original fish, thus producing more hybrid fish that don't necessarily look or act like they should. This can be very annoying to a breeder who may have spent a lot of money purchasing the stock and then has to destroy all their fish because they don't know which one is the problem and produces the mutants.
Most shops won't buy hybrids so if you do cross different species you should only keep a few for yourself and not try to sell or give them to anybody.