I am having a hard time trying to un screw my bulk..under my tank...it's been there for bout two years and I can get it to move....I don't want to damage it..
Spray some hot water on it. It will loosen up the salt creep, and also warm up the plastic. Is your tank empty? or are you going to cause a major flood when you take it off? (just kidding!)
did you put the bulkead on, and are you sure it's not glued?
This is one reason why I'm not in favor of having tanks drilled from underneath. If the bulkhead is connected to the tank be careful as I've seen people crack tanks trying to turn the bulkheads.