Abstract: An air flow rate regulation apparatus is used for regulating a flow rate of discharge air of an air pump used for breeding aquarium fishes. The apparatus includes a cylindrical control member having a hollow chamber defined thereinside and a regulation hole formed in the outer peripheral surface thereof and a cylindrical regulation member having a regulation recess overlapping with the regulation hole and made of a flexible material. The regulation member is rotatably fitted over the outer peripheral surface of the control member so as to adjustably cover the regulation hole, and the overlapped area of the regulation hole and the regulation recess is increased or decreased by rotating the control member and the regulation member relative to each other so that the overlap area can be continuously increased or decreased over a broad range of the rotating angle, 90.degree. or larger, of the control member and the regulation member.