A puffer will puff up when it sences fear. That is it defence. It is not a good thing. You should never use a net to catch them and always keep then under water. If they blow up with air, it is very hard for them to get rid of it. I saw one being brought back to my lfs that had blown up and they had to help it get rid of the air. Make sure you feed them a lot of meaty food, and give them some hard foods to keep their teeth filed down, Good Luck