Be sure to take account that a 3/4 diamond tip hole saw cuts to inside diameter. . Some 3/4 bulk heads I have seen have about 1-3/8s where the threads go through the tank.. lowes diamond tip hole saws from 35 to 50 roughly.
A 3/4" bulkhead is the INNER diameter of the piece. It does not include the outer diameter OR the threads. If you are drilling a hole for a 3/4" bulkhead, the hole should be around 1 3/8".
Get a new one from eBay. They are cheap and maybe only last for ~20 holes, but that should be enough. Drilling small tanks with thin glass is risky - go slow and be careful.
I used pvc schedule 40. 3/4 bulkhead for the threads of the bulkhead to fit nicely I had to use a 1-3/8" bit.. but yeah... what ever material ya use will most likely be different considering the thickness of the material used.
