Originally Posted by promisetbg
You could do it easy in a 300. Keep in mind that they are messy eaters and all puffers need to be fed shelled food items on a regular basis for their ever growing beak.
Considering that Diodon hystrix will reach 36" in length, a 300gal. would NOT be ok. Even a 300gal. that's 36" wide, as boalgf suggested, would still not work.
Even something on the scale of 600gal. would not work, IMO.