Marine aquaria include "fish only" tanks, "fish only with live rock" (FOWLR) tanks, and "reef tanks". Fish only tanks often showcase large or aggressive marine fish species and generally rely on mechanical and chemical filtration. FOWLR and reef tanks use "live rock", a material composed of coral skeletons harboring beneficial nitrogen waste metabolizing bacteria, as a means of more natural biological filtration. A reef aquarium or reef tank is an aquarium that prominently displays live corals and other marine invertebrates as well as fish that play a role in maintaining the coral reef environment. A reef aquarium requires appropriately intense lighting, turbulent water movement, and more stable water chemistry than fish-only marine aquaria.