An aquarium comprises a container, a dividing plate and a filtration system. The container includes a main cavity for containing water and live underwater creatures, and a bottom cavity below the main cavity. The dividing plate separates the main cavity from the bottom cavity and provides a barrier against fluid communication between the main cavity and the bottom cavity along all but a small fraction of the length and depth of the container. The filtration system communicates by fluid with the bottom cavity for intake of polluted water and with the main cavity for outgo of purified water after the polluted water has been purified by the filtration system.