[NATIVEVAMAN]

A hole of 1/8 or so shouldnt effect the volume in the overflow all that much.How about cutting off a 1 inch piece of the same size pipe and then cut that lengthwise and glue it over the whole? If you have an elbow in your return line just where it goes back into the main tank you should drill the whole in the bottom of that elbow pointing down. In theory that elbow should be above the main tank water level. If the pipe is glued in place this will be very difficult to accomplish and you may want to drill the new hole in the back of the fitting above the water level and squirting back into the overflow and hope it doesnt splash up on your lights.Its always the smallest problems that go unforseen and cause the biggest pain the ***.