I would use pvc through the wall make sure you don’t hit a stud or electrical when you drill through. What I would do is take your overflow run it into your frag just like you do in your sump take both hoses and put into top of your frag tank. That is assuming your main display is taller than your frag tank. Then just run your overflow from your frag tank down to sump and just keep your current pump to display tank set up. I have done this before works well but what you need to think about is what happens when you lose power will sump hold the drain from display and frag tank without over flowing if you get suction back from the display and depending on how low you drill the return hole on the frag tank. Get your bulkheads from LFS or bulk reef ect. Just make sure it matches what size you want to use do use pvc through a wall that way you never have to worry about a leak as it’s rated for in wall use. Drill the acrylic it’s easy I wouldn’t even drain whole tank just lower water 4 or 5 inches drill from outside have a second set up hands holding plastic or something to catch.