Like Geoff says it depends on what type of BH you use, most you will need a 1 3/4" diamond hole saw for a 1" bulkhead.
I have all my tanks drilled in the upper corners with the bulkheads and then have elbows with strainers on them so I have no ugly OF box in the tank. I get great flow like this.
Most tanks have a tempered bottom so I would not try that. Check with the company that makes the tank to make sure your tank is not tempered. I had a 55 drilled at the LFS and it burst when the hole was about half done. It was tempered 
I drilled a 37 tall and 90 about a month ago, the key is keeping the bit cool at all times. use plenty of water or the special glass drilling fluid.
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9/02- 125 Reef, 4x160 VHO, Reef devil skimmer, 130 lb LR,LS, 15 gal tall fuge.
7/03- 55 FOWLR, 4x65 PC, 60 lb LR, South Down, Sump, RD skimmer, 20 gal fuge.
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