I think best to error on safe side. I will drill another 1 in. as planned on rear, giving me the 4 holes. Im going to drill it a bit more to the open side of the tank and not closer to the side glass. I will just enlarge my overflow to 1ft. Takes away more tank but I will have a duzer of an over flow and herbie/beanie thing or whatever its called. :mrgreen:

you could do it fairly easy, just need one of these, and yours drill bits have to fit it.

http://www.leevalley.com/en/wood/pag...at=1,180,42316

most diamond hole saws have built in shafts though so might not work.

other option is to just drill around it, if you start on an angle and then shift to flat it shouldn't be to difficult, just use less pressure than normal and take your time. other wise get a new chunk of glass and redrill it, (this is on your overflow is it not) would be cheep for a small chunk of glass.

Steve

Im pretty sure just going to drill another 1 in hole in the back Steve. That will give me 4 as above. I ordered a larger overflow glass. Its now 12 in by 6in and 17.5 H not including eggcrate

Its a loss of space for the larger overflow but the safest way to go. And with the 4 I can do a 3/4in. return through one of the smaller bottom holes.

So a 1in. main full drain line...........a 1in. air intake 2nd drain line..........a 1in. but bumped up from a 1/2in. bulkhead, safety drain......and a 1/2in. bulkhead bumped to 3/4in. return line which is a perfect for for my Mag 12