drilling 100 gall established?

i have a 100 gallon fish only that has a 3/4 diy pvc overflow the flow raes it can handle are very lame. i want to drill it but i dont know how drilling with livestock would be. and the hang on back overflows you buy just seem like a waste of money if im still risking a flood. what are my options. can i just lower the water level and drill will the fish be ok one day not connected to their sump?