Penn-Plax Check Valve/Air Filters are perfect for use with your aquarium air pumps & accessories. The air filter provides clean air to your pump removing small particulate matter that may clog air stones or other air devices. The check valve engages whenever your air pump is off or in the event of a power outage stopping water from siphoning back into your air pump and preventing possible water damage. The check valve also makes adding and removing devices safer and can save you the cost of replacing an expensive device or changing contaminated air lines.