Puffers tend to be aggressive and usually do best kept alone. I have never had any luck feeding flake or pellet to puffers. Puffers need space - give them the largest tank you can.
Puffers are sensitive to organic waste build-up in the water.
They do best with weekly partial water changes. If your puffer is a brackish variety, keep your salinity level constant. Use a good synthetic sea water mix and remember salt does not evaporate.