A tiny hole is all that is needed, something like 1/16". If your loc-line is near the top of the tank, you might not even need a hole. Your system should have enough sump capacity to hold whatever drains out of your display tank in the case of a power outage. Meaning that you could drain your tank to just below the drains, and not have a flood. So keeping the output of the locline at or above the drains would eliminate the need to drill a hole.
I wouldnt count on a check valve to prevent a flood. The amount of gunk that builds up even in high-flow plumbing is enough to keep a check valve from properly sealing in no time at all.