Ok, yes check valves are not the option. They're not trustworthy and will drastically increase head pressure on your pump.
The best solution is the anti-siphon hole. My return pipes are made from all-glass. It's a very simple design. Right at the sharpest bend of the 90 degree angle at the top of the pipe, there is a small 1/8" hole under the water. Once I shut my return pump off, the water falls and then reaches below this hole to grab air. Siphon is now broken... HOWEVER, this can easily just clog up with salt creep, a chunk of debris, anything, that your return pump might blast back up to it... This is a very simple thing to check. take a big fat needle in your hand, stick your hand in the water where you know the hole is, and put the needle in and wiggle it around... Now you know you're good... You just have to remember to do this every so many months or weeks. Takes 2 seconds to maintain... I had mine clog once. No biggie. I noticed it when I was doing a water change instead of during a power failure before the sump got too high.
