cant remember off the top of my head, but i used the GE silicone that was "food safe". type 1 i guess? cant remember cuz i'm not at home.
rev, did you get the bulkhead fitted yet?
if i had that problem, i would have just gotten a diamond encrusted bit for a dremel and used the dremel to widen up the hole a bit.
where did you buy your diamond hole saws from?
btw...premium aquatics should have some cheap bulkheads in. also, there are some really awesome/bulky ones on mcmaster carr's website.