Abstract: An aquarium for jellyfish and other plankton species is described. The aquarium has a water container formed by two vertical front facets and a side surface between the front facets, the container having a polygonal, round or other shape when viewed from the front. A sealed chamber is located adjacent the water container with a water inlet that provides water to the chamber. A diffuser block provides a water passage from the sealed chamber to the water container. The diffuser block has a plurality of parallel channels so that water entering the container from the sealed chamber forms a plurality of jet streams along the side surface. This creates a laminar flow of water inside the water container near the side surface, which helps to keep the jellyfish suspended. The water container also has a discharge port covered with a screen to prevent the jellyfish from being sucked out.