Fishkeeping is a popular hobby, practiced by aquarists, concerned with keeping fish in a home aquarium or garden pond. There is also a piscicultural fishkeeping industry, as a branch of agriculture.
Ancient Sumerians kept wild-caught fish in ponds, before preparing them for meals. Depictions of the sacred fish of Oxyrhynchus kept in captivity in rectangular temple pools have been found in ancient Egyptian art.
Freshwater fishkeeping is by far the most popular branch of the hobby, with even small pet stores often selling a variety of freshwater fish, such as goldfish, guppies, and angelfish. While most freshwater aquaria are community tanks containing a variety of compatible species, single-species breeding aquaria are also popular. Livebearing fish such as mollies and guppies are among those most easily raised in captivity, but aquarists also regularly breed many types of cichlid, catfish, characins, cyprinids, and killifish.
Marine aquarists often attempt to recreate a coral reef in their aquaria using large quantities of living rock, porous calcareous rocks encrusted with coralline algae, sponges, worms, and other small marine organisms. Larger corals, as well as shrimps, crabs, echinoderms, and mollusks are added later on, once the aquarium has matured, as well as a variety of small fish. Such aquaria are sometimes called reef tanks.
Ideal aquarium ecology reproduces the balance found in nature in the closed system of an aquarium. In practice, it is virtually impossible to maintain a perfect balance. As an example, a balanced predator-prey relationship is nearly impossible to maintain in even the largest aquaria. Typically, an aquarium keeper must actively maintain balance in the small ecosystems that aquaria provide.
Home aquarists typically use modified tap water supplied through their local water supply network. Because of the chlorine used to disinfect drinking water supplies for human consumption, tap water cannot be immediately used. In the past, it was possible to "condition" the water by simply letting the water stand for a day or two, which allows the chlorine to dissipate. However, monochloramine became popular in water treatment because it stays longer in the water. Additives are available to remove chlorine or chloramine and suffice to make the water ready. Brackish or saltwater aquaria require the addition of a mixture of salts and other minerals.
In a planted aquarium, aquatic plants also metabolize ammonium and nitrate as nutrients, removing them from the water column primarily through leaf surfaces. Plants remove some nutrients through their roots, either in or at the substrate level or via aerial roots floating in the water. Additional nitrogen and other nutrients are also made available for root uptake by decomposing organic matter in the substrate as well as the breakdown of mulm. While very small amounts of rotting foliage may be allowed to decompose and cycle nitrogen back into a planted aquarium, in practice aquarists will prune and remove substantial amounts of plant litter.
Experienced aquarists warn against mechanically applying these rules because they do not consider other important issues such as growth rate, activity level, social behavior, and such. Once the tank nears capacity, the best practice is to add the remaining fish over a period of time while monitoring water quality.
Historically, fish and plants for the first modern aquaria were gathered from the wild and transported (usually by ship) to Europea and America. During the early 20th century many species of small colorful tropical fish were exported from Manaus, Brazil; Bangkok, Thailand; Jakarta, Indonesia; the Netherlands Antilles; Kolkata, India; and other tropical countries. Import of wild fish, plants, and invertebrates for aquaria continues today around the world. Many species have not been successfully bred in captivity. In many developing countries, locals survive by collecting specimens for the aquarium trade and continue to introduce new species to the market.
Hybrid fish such as flowerhorn cichlids and blood parrot cichlids are controversial. Blood parrot cichlids in particular have a very unnatural shape that prevents them from swimming properly and makes it difficult for them to engage in normal feeding and social behaviors. The biggest concern with hybrids is that they may be bred with native species, making it difficult for hobbyists to identify and breed particular species. This is especially important to hobbyists who shelter species that are rare or extinct in the wild. Extreme mutations have been selected for by some breeders; some fancy goldfish varieties in particular have features that prevent the fish from swimming, seeing, or feeding properly.