The big problem with check valves in a marine system is there is all sorts of growth everywhere! Almost all of it has some form of calcium carbonate structure. So stuff grows on the check valve seal. The stuff is hard like grains of sand. So after some time, usually when you are not home (Murphy and his damm law!), there is a power failure and the tank drains till there is an air gap because the seal can no longer form a water tight connection. Make sure your sump can handle the drain down! Don't depend on the check valve! Another issue is the CV eats up return flow!