Waikīkī Aquarium

These may be either freshwater or saltwater, and are supposed to display a tiny however self-contained ecosystem. Reef aquaria under a hundred litres (26 US gal; 22 imp gal) have a special place in the Aquarium hobby; these aquaria, termed nano reefs , have a small water volume, under 40 litres (11 US gal; 9 imp gal). In a real kreisel, a round tank has a circular, submerged lid. Pseudokreisels are “U” or semicircle shaped, usually with no lid. Stretch kreisels are a “double gyre” kreisel design, the place the tank length is at least twice the height. Using two downwelling inlets on either side of the tank lets gravity create two gyres within the tank.