A vent valve of an air pump typically utilized in an aquarium apparatus. The vent valve includes a wall member with a vent hole in the wall member. A valve body having a flap member closes onto the vent hole. The closure action of the flap member is regulated so that the flap is twisted under the force of air flowing through the vent hole in a direction toward the flap member and returns to a closed position against the wall member with a rolling sidewise fold-down motion when an air flow lessens. In this manner, there is a reduction in the flapping noise of the valve.
