Use some pliers or vise grip pliers, but ... also use some form of towel or rag in between the pliers and piece. Then, you night need to re-die or re-tap it to get the threading back correctly. You may have cross threaded the piece and in that case have stripped the threading. So your post reads as best to my experience. "That's what it sounds like to me without looking at it hands on." I used to do similar in tightening stuff too tight.
Oh yeah, be gentle but also be firm and pull the piece free. You don't want to damage it but you do want it tugged off or out of the tank. That's why I suggest padding w/ towel/rag.