Sometimes there is a small amount of what is basically plumber's putty on the fittings. In theory, there should be no need for any kind of sealer, since these are pipe fittings. Pipe fittings, unlike regular threaded stuff, have a slight taper tot he threads. The farther in the fitting goes, the tighter the fit...in theory. A little pookie will certainly help minimize the chances of a leak. I would suggest a small amount of 3M Aviation sealer, or, if you happen to have some handy, regular pipe dope.
You'll need a wrench that fits nice and snug on the fitting, and enough room to swing the wrench. Depending on what your fitting looks like, you might even be able to get a socket on it. Work it slowly at first, to determine if it is really tight, or just barely in there. If it is tight, you may want to tap on the top of it with a small hammer to try to break it loose a bit. Most of them are not much more than hand tight, but you never know....someone may have gotten a bit ham-fisted, and screwed that thing in with a cheater pipe....
hth
______________
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"