I used to have a Fugu Puffer that did that when he got scared. But after a while he got so aggressive that when you got close to the tank he would bite at the glass and one time I went to feed him a live fish while some friends were watching and he jumped out of the water and put a hole in my net that I could stick my thumb through, taking half the fish with him. The one guy thought it was so cool he bought him from me.