As others have suggested, run your return line up and over the top of your tank..
And then maybe drill a couple of holes (into your return nozzles) just below your normal water line for a siphon break. If your sump is big enough you don't need siphon break holes as the return nozzles will break the siphon once the water drops down below the top of the return nozzle, allowing air to enter.
Here are my siphon break holes. The water has drained down to these holes and no further. During normal operations the water level is 1 inch above these two holes.
Even without these holes the water only drains about another couple of inches before it reaches the top of the nozzle where air will enter and again break the siphon.