Pumps that are placed too close to the surface, move too many gallons per hour, or some combination of the two can lead to the pump creating a vortex and sucking in air, or causing cavitation. The air will get sucked into the pump, causing the pump to make a not too pleasant noise and shoot bubbles all over your aquarium. You could try putting the pumps deeper into your aquarium if this happens, or you could fashion some form of plastic shield to go over a portion of the intake. If the problem still persists, it might be time to get a smaller pump.
Tip of the Day – 9/30/20100