You really should not have any problems breaking tanks... I just spin the drill at a fast to moderately fast speed, try and keep the area wet with water, and use very little pressure against the glass. Also NEVER try and drill on tempered glass! Usually the tank will have a sticker on the pane of glass if it has been tempered. Also if you are concerned you can call the manufacturer and ask which panes of glass on your size tank are tempered.
Good luck and have fun.
