bulldog188
Fish Fanatic
I've found when stressed etc they curl their tails to one side with the fin remaining closed. the fanning, as you said, is just to aid movement etc. I've got a 3" fahaka puffer in a 200L tank with perfect water etc.. he's the only occupant so no stress etc [mainly cos he'd kill anything that broke the surface of the water ] and he often fans his tail out as he swims the length of the tank..