An aquarium is an enclosed tank that holds water, aquatic life, and often plants. The hobby of maintaining these systems is called fishkeeping, and it spans everything from small desktop bowls to large living-room reef installations. The two broadest divisions are freshwater and saltwater tanks. Freshwater setups contain no salinity and are generally simpler, while saltwater tanks require sea salt mixed to a precise specific gravity and demand more complex equipment. Between these extremes sits brackish water, which has a low salinity somewhere between fresh and salt and is used for species from estuaries and mangrove habitats.
Beyond water type, aquariums fall into several major categories. A community tank combines peaceful, compatible species. A planted tank prioritizes live aquatic plants as both decoration and biological support. A biotope replicates a specific natural habitat, such as an Amazonian stream or a Lake Tanganyika shoreline, often using fish and decor from that same region. A species-only tank houses a single species, usually for breeding or specialized care. In the saltwater world, a reef tank is built around corals and other reef-associated life, distinct from a fish-only saltwater setup.
For newcomers, one principle shapes every choice that follows: bigger is more stable. A larger volume of water resists swings in temperature, pH, and waste concentration, so a 20-gallon or larger tank is almost always easier to manage than a smaller one. Beginning with a sturdy starter setup of tank, filter, heater, light, substrate, decor, and dechlorinator allows the hobbyist to focus on learning rather than rescuing a fragile system.