Puffer fish are probably the hardest to catch in Zen Koi. Even though they swim really slowly, and don’t turn quickly, they’re really tricky to catch.
Whenever you try to swim to them, they inflate, and cannot be caught.
How to catch Puffer Fish:
The trick is to swim behind them, and sneak up where they can’t see you coming. If you approach at any angle that’s not exactly behind them, they will spot you and inflate.
If you do happen to get the angle wrong, just pause for a moment and the puffer fish will swim forward a little and deflate. Most of the time, you’ll then be in a good position to sneak up on it.
Edit: Some readers have suggested holding your finger down and lead your Koi to eat the puffer, rather than tapping on the puffer fish directly. This seems to make the puffer less likely to “puff”, but you’ll still need to approach stealthily from the rear.