Tropical fish include fish found in tropical environments around the world, including both fresh water and salt water species. Tropical fish are popular aquarium fish , due to their often bright coloration. Tropical fish kept for home aquaria include the following: * Wild-caught specimens. * Single-species individuals born in captivity. The latter category includes lines selectively bred for special physical features, such as long fins, or particular colorations, such as albino. * Hybrids of more than one species. Recreational SCUBA divers are often enthusiasts of tropical fish as well. Some keep lists of fish species they have observed while diving, especially in tropical marine environments.*************An aquarium (plural aquariums or aquaria) is a vivarium, usually contained in a clear-sided container typically constructed of glass or high-strength plastic, in which water-dwelling plants and animals are kept in captivity, often for public display; or it is an establishment featuring such displays. Animals in aquariums usually include fish, sometimes invertebrates, as well as amphibians, marine mammals, or reptiles. ********************** Aquarium keeping is a popular hobby around the world, with about 60 million enthusiasts worldwide. From the 1850s, when the predecessor of the modern aquarium was first developed as a novelty, the ranks of aquarists have swelled as more sophisticated systems including lighting and filtration systems were developed to keep aquarium fish healthy. Public aquaria reproduce the home aquarist's hobby on a grand scale — the Osaka Aquarium, for example, boasts a tank of 5,400 m³ (1.4 million U.S. gallons) and a collection of about 580 species of aquatic life, whilst the planned National institute for research into aquatic habitats in England would be at 40-hectares the world's largest aquarium. A wide variety of aquaria are now kept by hobbyists, ranging from a simple bowl housing a single fish to complex simulated ecosystems with carefully engineered support systems. *********************** Aquaria are usually classified as containing fresh water or salt water and brackish water, at tropical or cold water temperatures. These characteristics, and others, determine the type of fish and other inhabitants that can survive and thrive in the aquarium. Inhabitants for aquaria are often collected from the wild, although there is a growing list of organisms that are bred in captivity for supply to the aquarium trade. The careful aquarist dedicates considerable effort to maintaining a tank ecology that mimics its inhabitants' natural habitat.