boy, lots of things to respond to.
So, yes i
saw the 3/4" is limited to approximately 350 gph vs the 600+ gph of the 1" bulkhead which is going to suck.
About gasket, yes it is on the inside. To be honest, it was fine until I cut my plumbing in order to plumb in the new frag tank i installed in march of this year. When I cut through the pipe, I heard a crack and honestly thought I had cracked the glass of the tank because there was a bunch of water flowing from the bulkhead. After re-tightening the bulkhead I was able to stop the leaking for the most part, but i am still seeing salt creep build up in the area so i know theres still a minor leak. I have checked as best as I can to see if there is an actual crack in teh bulkhead, but it doesnt seem like it.
About drilling a 3rd hole in the bottom, totally out of the question. If anything, I would think about making the 2 holes in there already drains and just plumbing a return pump up around the back of the tank to come out on the right side which would also help increase flow throughout the tank.
The current setup I have with the 1.5" pipe reduced to the 1" bulkhead is what came with the system when I purchased it used. I have read a little that says that it causes more head pressure so forces more flow through the bulkhead, although im not so sure I believe it. One of the main reasons I would do the 1.5" down to the 3/4" with the new plumbing is just so I can reuse the length of tubing already there for the drain line.
Another possible option is to use the schedule 80 for the return line, but have the drain remain the 1" abs crap-o bulkhead.