Marine aquarium

A marine aquarium is a contained environment for saltwater plants and animals. Hobbyists typically classify these tanks into three types: fish only (FO), fish only with live rock (FOWLR), and reef aquariums.

FO tanks often house large or aggressive fish and rely on mechanical and chemical filtration. FOWLR and reef tanks use live rock—coral skeletons that host beneficial bacteria for nitrogen waste processing—as a natural biological filter. Reef aquariums feature live corals, invertebrates, and fish.

Saltwater fishkeeping differs fundamentally from freshwater due to the distinct composition of saltwater and how its inhabitants adapt. Marine systems demand more equipment and stricter water quality monitoring than freshwater setups. Acquiring marine inhabitants is often harder and more costly.

Marine fishkeeping history

The first saltwater tanks were Roman outdoor jars for anemones, but they lasted only briefly. The Aztec Empire maintained ten saltwater ponds at Texcoco. In 1846, Anna Thynne kept stony corals and seaweed for nearly three years in London, credited as creating the first balanced marine aquarium.

Personal saltwater fishkeeping expanded in the 1950s with basic 20-gallon glass tanks. Bleached coral and coarse crushed coral were standard substrates. Algae, including beneficial coralline algae, were removed, as a clean, sterile tank was considered healthiest.

Early hobbyists collected seawater from local beaches, which contained unwanted organisms and pollutants. Common fish included percula clownfish, sergeant major damselfish, small brackish-water pufferfish and scats, jeweled blennies, and blue damsels.

Tanks used large air compressors, heavy aeration, and undergravel filters. The inconvenience of gathering natural seawater, along with advances in analytical chemistry, led to synthetic salt mixes that replicate the tropical ocean’s chemistry, including trace elements. This made marine fishkeeping popular where clean seawater was unavailable.

Germany produced air-driven protein skimmers and reliable submersible heaters. Filtration advances like trickle and hang-on filters helped maintain natural equilibrium. Improved fluorescent and metal halide lighting enabled the first reef tanks, allowing corals and invertebrates to thrive without sunlight.