lets just say you have your sump set up this way
| (inlet) skimmer | bubble trap | return pump | bubble trap | refugium (inlet) |
the slightly over rate pump is allowed to pump straight back up to the tank but the "t off" has a ball valve which when fully open slows the rate of the pump and when fully closed (which i dont recommend on an overrate pump = flood ) will speed up the pump. this was a 30 second sketch in paint but if you need further explanation please let me know.
as for drilling a tank, first your going to want to know if the tank is tempered. in alot of cases the back glass will not be but to be safe, contact the company of purchase. i personally used www.glass-holes.com
as their kits come with a drill bit and overflow. there are other options but this is what i did for my tank and like the outcome. just having a hole, will most likely be noisy, not help skim the surface of the water ( like a box does ) and will suck down snails and even fish ( any over flow has this risk but just a hole increases this risk ) also if your pump fails or the power goes out the water will siphon down until it gets air (siphon break) of the hole drilled. this means if your hole is 2 inches below the water line, 2 inches of water will drain to the sump, it even siphons back down the return lines unless you have siphon breaks. just all things to consider.