This is a good example of why it's so important to research BEFORE buying. Puffers eat shrimp, crabs, etc. alot of what we want to keep alive. Also they are aggressive and will nip and potentially kill tankmates.
Also it's important to remember alot of aquarium store employees don't know what they're talking about. Sorry, but it's true. Most stores are guilty of it. Any store that says a Puffer can live in a 40g with corals and inverts and eats frozen mysis is dead wrong! It is SO important to do your homework, as it's up to you to decide whether the selection is appropriate. The only one who suffers here is the poor fish.
Puffers need to be fed big meaty foods. Shrimp, Krill and Squid are vital.