If your tanks are mass production tanks they are a real pain to drill and you can get it done but I dont recommend it. What I do recommend is getting some 1/8" thickness poly vinyl sheet and using about a 4"x4" or whatever size piece, cut a hole the size you want the hole in your tank to be and then masking tape or duct tape or glue it in place and then take your tank to any local shop that has a bead blast or sand blast unit and get them to bead blast a hole through the glass, the poly vinyl will protect the area around the hole and you will be left with a nice clean hole that has smooth edges that wont be prone to chipping and cracking. This is my favorite method of putting holes into pre made tanks and the only way to "drill" tempered tanks.