The overwhelming majority of tropical fishes seen in shops are bred in tanks or ponds and may represent forms that are unknown in the wild. Free from the pressures of predation and competition, albino and long-finned forms of many species have been bred and would not survive in a wild situation. Due to environmental damage, a number of species familiar to aquarium keepers are threatened or even extinct in the wild such as the Redtailed Black Shark (Epalzeorhynchus bicolor) or the Cherry Barb (Puntius titteya) and for these, popularity as aquarium fish ensures their survival.
A much smaller proportion of fishes in the trade consist of those that are collected from the wild. In the case of freshwater species, many of these are harvested from areas where massive seasonal fluctuations in water levels mean that a surplus of individuals are destined to die in the dry season and represent a very sustainable means of income for local people. As such, these small fishes remain a valuable commodity to be protected and with them their habitat - ensuring the preservation of biodiversity and discouraging destructive land use. As long as the species are those that will thrive under captive conditions, such sustainable use of a renewable resource is to be encouraged and in these enlightened times valuable work in nature reserves can be financed by the judicious collection of wild-caught fish.
Unlike their captive-bred counterparts, wild fish are used to optimal water conditions and will often prove problematic if your aquarium water is high in pollutants such as nitrate or phosphate. Make sure that these levels are acceptably low before introducing any new fishes to your aquarium.