I have a single check valve on my return line. I also have ball valves on both in and out lines just in case. I wouldnt rely just on a check valve though, they can fail. Make sure you put a small hole in the return line where it is just under the water in the tank so that if the check valve fails at least the hole will break the back siphon.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.