Check valves are not designed for reef use. They can and will fail almost guaranteed. Remember it does not have to be a catastrophic failure, even something as simple as a grain of sand or a tiny snail can defeat it and that rickle will flood given time, usually when you are not at home. A piece of algae, an anemone, a snail, some foozen food, a small fish, anything can get in there.
On the simple side, an air gap cannot flood since we all know water cannot jump up hill. Leave the returns say 1" below the surface of the display and you can sleep soundly at night knowing only a couple gallons, very easily calculated, can possibly return to the sump in a power outage before the returns are exposed to air and the siphon quits. To calculate its simple L x H X W /231. If I had to guess I would say your tank is 72" long and 18" front to back so 72x18x1" if you lower the returns 1" below the surface / 231 = 5.6 gallons. So a maximum of 5.6 gallons could possibly backflow is all and your sump should have more than that for spare room when in operation since the protein skimmer will only be submerged 8 or 9 inches. With my 100G tank it is 60" x 18" x 3/4" which is where my drilled returns and Loc Line are placed below the surface so 3.5 gallons maximum in a 30G sump is easy to contain. No drilled holes in the returns to clean or listen to and no check valves to add to headloss, require cleaning and fail and I have no problems sleeping peacefully at night knowing there is no possibility of a flood.
