Abstract: An aquarium having a tank with a flat, built-in aerator mounted at the rear. The aerator includes a housing having a number of narrow internal baffles arranged to form a zig-zag passageway for the upward flow of air into a space in the top of the housing. The bottom of the zig-zag passageway is disposed above a clear space in the lower part of the aerator housing, and each of the baffles runs from a first end at a first side edge to termination short of the opposite side edge of the housing. The baffles extend alternately from the opposite sides of the aerator housing, and each slopes slightly upwardly from its first end to its other end within the housing. The baffles are so arranged that each adjacent pair defines a transverse leg of the zig-zag passageway extending from one side of the aerator housing to the other and diverging vertically from a restricted entrance end to a far end where the entrance to the next higher leg of the passageway is situated.