Tropical fish are popular choices for aquariums due to their often bright coloration, which typically derives from both pigmented cells and iridescent cells. Tropical fish may include wild-caught specimens, individuals born in captivity including lines selectively bred for special physical features, such as long fins, or particular colorations, such as albino. Some fish may be hybrids of more than one species.
Many marine tropical fish, particularly those of interest to fishkeepers, are those that live among or in close relation to coral reefs. Coral reefs form complex ecosystems with tremendous biodiversity. Among the myriad ocean inhabitants, the fish stand out as particularly colourful and interesting to watch. Hundreds of species can exist in a small area of a healthy reef, many of them hidden or well camouflaged. Reef fish have developed many ingenious specialisations adapted to survival on the reefs.
Coral reefs occupy less than one percent of the surface area of the world oceans, yet they provide a home for 25 percent of all marine fish species. Reef habitats are a sharp contrast to the open water habitats that make up the other 99% of the world's oceans.