Do you have a sump also? If so, you can set it up like I have mine - I divert a little bit of the flow from my return pump to the refugium (which is next to my tank), then I have the refugium drain back into the sump.
IF you don't have a sump, then the next best thing would be to elevate the refugium slightly higher than the tank, drill a bulkhead into the refugium as close as you can to the top, then use that to drain into the main tank. Use a powerhead or a small pump like a Mag3 to pump water to the refugium.
If you want the levels exactly the same (and don't have a sump), then you will have to use a Utube to return water to the main tank (you still need a small pump to get the water into the refugium). You would want to design it like a good overflow (Lifereef, etc.) where you have a box inside each tank with a small compartment that keeps the UTube submerged. Otherwise you would be guaranteed a flood when you have a power failure and lose the siphon in the UTube.
