I wouldn't worry about the tail fin spreading thing..just a form of communication I'd say, but not bad.. They are great little fish and are about the best personalities I have ever seen. Mine found a spot on the powerhead in which he sleeps every night, right after the lights go out... and if I turn them out a little later, at times he has already gone to bed, an hour before. He lays absolutely motionless on the powerhead all night. He sleeps pretty soundly too. I have had my hand in the tank in the morning and accidently brushed him off off the powerhead. He would slowly fall down the tank motionless until he landed on the sand or a rock, then slowly start swimming for about 20 seconds before he would awaken and start out the new day. He eats just about anything and tends to eat until you can actually see his tummy is full. I never have seen him be agressive towards any other fish or any corals. He occasionally will pick at a rock, but that always just seems to be occasional algea picking.
One thing I will mention is that mine is a Saddled Filefish or Valentini mimic (Paraluteres prionurus) which is almost always sold as a 'Valentini Puffer'.
A true Valentini Puffer, is the Saddled or Valentini Toby (Canthigaster valentini) ...(poisonis to it's preditors)..
They look almost just alike... Photos can be seen with a discussion in the Book:
The Conscientious Marine Aquarist
by Robert Fenner
Pages 317 & 320
Great Book, by the way, and is available through our Host under Dry Goods -> Books.