An automatic aquarium feeder apparatus including a body unit having a platform element; a platform wall on the platform element defining a cavity; a clamping mechanism for clamping the platform element to the walls of an aquarium; a drive mechanism including a battery-powered motor; and a food dispensing element having one or more first slots through the platform element, and a disk element rotatably driven by the motor and having one or more second slots that are alignable with the one or more first slots. Food in the cavity gravitationally drops through the first and second slots when aligned. The drive mechanism may include an alternating power on/power off timing mechanism.