Pufferfish will “puff up” as a defense mechanism if they are threatened. A shape that is more than double its original size, round and sometimes covered in spines is much more difficult to bite and isn't very appetizing to a predator.

Is it bad for puffer fish to puff up?

Unfortunately, while puffing up can successfully discouraging predators, it also can be quite harmful for the pufferfish, sometimes leading to death.

How do you get puffer fish to puff up?

Tap the puffer with the handle of the net. It will probably just swim away. Follow the puffer with the net, continuing to tap it. This will eventually either scare or anger the fish into puffing up.