Fella put it very nicely. Pufferfish in the wild graze continuously. What they eat is primarily shelly, so can be thought of as being "poor quality" food (what biologists call food with a high ash content, i.e., a lot of indigestible stuff). What they do is fill up with poor quality food, poop out most of it, and have to extract what nutrition they can.
In the wild we do the opposite for them. We give them high quality food. While they quickly get all the nutrition they need, their instinct to feed continously is very strong, and for that matter to gorge themselves while they can.
Most fish only need a tiny amount of food. Because they are cold blooded, they mostly need energy for growth and repair. Warm blooded animals use something like 80-90% of their energy for maintaining a constant body temperature. That's why a snake can get by on one mouse a month, where a cat of the same body mass would need one or two mice every day.
It's almost impossible to underfeed a fish. A two-inch pufferfish (I would guess) needs only one or two small snails a day, or 4-5 bloodworms.
Cheers,
Neale