I have drilled about 40 holes and they were all fine, but i finally messed up on my own tank. I picked up a Freshwater Waterbox Eden 40 locally for really cheap, $250 for a clean tank, Ai Blade and stand. So of course the first thing i did was test for leaks and it passed, so i went to drill for a 1.5" overflow bukhead and it cracked on me. I did every i normally do and clamped a piece of plywood underneath to prevent a blow out. I am soo disappointed in myself.


. I took mine apart and got the glass cut and made my own sump. Just take it apart don't try and save the rim just hack saw it apart and clean the glass with alcohol ( you can drink some )
. Kidding . Wrap it in cardboard store it . Make a Sump its a fun project. As to the crack. I bought another tank let a glass place do it . I used the same method as you even used a drill guide felt the one side go through so did a little tilt . WRONG ! the glass guy put the tank on its side on plywood and it went perfect even with the gap from the rim. Go figur