Just unscrew the bulk head from the tank and buy new one. I have seen old bulk heads crack when you try to screw stuff into them. Bulk heads are cheap to buy depending on the size, cleaning 500 gallons of water off the floor is not fun, been there down that. Just unscrew the old one, scrape any silicone off and put new bulk head in. I use a little bit of PL roofing/flashing sealant made by loctite, can be found at lowes in the caulk section. A lot of people use in ponds on bottom drains and sealing around bulk heads when going through liner. I just put a little around the bulk head on the inside once you have it all tightned up. This sealant is black, if you want clear go with aquarium sealant, GE I, or RTV108.
Nice build so far.