Why do porcupine puffer "puff up"?

[sixers3]

I have a porcupine puffer in a 55 gallon tank by itself. It is a great fish, tons of personality, eats a ton, very healthy. Once in awhile I find him "puffed up" when I come upstairs. It seems to go along with him excreting. Is this a mechanism to help him excrete? Is it a way for them to grow?
Any help would be great.
Thanks.

 

[ophiura]

No it is a defensive or stress related behavior.

 

[rslinger]

Probably stressed from the small tank.

 

[seagoblin333]

It has to be stressed out. shouldnt be kept in smaller tank

 

[sepulatian]

This fish grows over a foot long and they are extremely messy. Have you checked your water quality? Please post your exact readings for ammonia, nitrite, nitrate, ph, SG, and temp.