Check valves create a one way route for air to enter your aquarium, but not return the opposite way. Most of the pumps in this review include check or return valves that will prevent back siphoning. Back siphoning occurs if the air pump is ever shut off, either intentionally or due to a power outage. If the pump is below the water line, gravity will force the tank water down the hose and spill into your unit. If not caught, you could accidentally siphon out your entire tank, leaving your fish high and dry! Check valves prevent this back siphoning from occurring but are not required. If you have a pump that does not include check valves, if you place your air pump above the water line, you will prevent back siphoning. Do not place it on top of your tank, however, since the vibrations will cause your fish to stress out and can cause excess noise in addition to water damage from evaporation.