Check a few things before buying new bulkheads. First,how sloppy is the fit in the drilled hole? It should be fairly snug with little side to side movement for best gasket fit. Next is it slip x slip, slip by thread or thread by thread? I prefer thread by thread for all my bulkheads personally. If its threaded take a good look at the threads and make sure they are absolutely free of any excess flashing or plastic scrap. Use a jewelers file, nail file, Xacto or pocket knife and clean the threads out on both the bulkhead and on the PVC that will screw into it. This one step alone fixes most drips since flashing makes teflon tape tear or roll out and not seal. On this subject I highly recommend the crayon stick type thread sealants instead of teflon paste or tape. Common brands are Plasto Seal made by LaCo found here:
http://www.laco.com/productDetail.aspx?ProductID=121
Or even Ace Hardware has a version like found here although this link is for 12 sticks.
http://www.acehardware.com/product/...ealant&parentPage=search&searchId=38557381993
If you insist on using teflon tape use only the tape designed for propane and natural gas piping, usually pink or yellow colored, its much heavier duty and its mill thickness is probably 10x thicker so only a wrap or two is required. It also does not smear, tear or roll out as easy as the white stuff.
Always install the bulkhead clean and dry, no silicone or sealants/lubricants on the gasket. As already pointed out the gasket always goes on the flange side regardless if its on the inside or outside of the tank. Hand tighten only and maybe 1/4 turn more at most with a pair of channel locks. Overtightening is a very common mistake.
Never support any weight with a bulkhead always support any piping or pumps with something so the bulkheads pressure stays even all around, another common mistake.
These are just a few of the things I have learned over the past 20 years in the hobby, some learned the hard way!
