I'd say a pufferfish is a bad choice of pet if you want something that will eat feeder fish. The reasons for this are that not only do they not have fish typically in their diet, puffers are so variable in personality some may not even eat feeder fish you give to them. Most puffers (especially those for a 29g or smaller) will be creatures that eat mainly snails, cockles, mussels, shrimp etc, not fish. Puffers also should be given a reasonably varied diet as well in my experience.
I'd think you'd have more choices on the puffer if you'd go brackish.
Where is the evidence to support this?
The puffers typically offered in stores are -
Figure 8 puffer
Green Spotted puffer
Ceylon puffer
Porcupine puffer
Dogface puffer
South American Puffer
Congo Puffer
Fahaka Puffer
Mbu Puffer
Irrubesco puffer
Lorteti puffer
Fangs puffer
etc
Only the top 3 are considered brackish, and the GSP and Ceylon is considered to need marine conditions in adulthood. There are plenty more puffers available that I haven't listed here that are freshwater (or marine) also, so to say brackish would increase the options is simply not true.
As for a single, aggressive puffer in a 30 gallon tank, the first thing I'd say is that this fish won't be big. Puffers are messy creatures, and when kept in tanks they need to have generous amounts of space to dilute the vast amounts of pollution they create. For a 30g you could keep -
"Auriglobus modestus" - The bronze puffer. This fish would have plenty of room in a 30, but they max out at 4". I
think this is one of the few puffers that is piscovorious, but I couldn't be sure.
"Carinotetraodon irrubesco" - the red eye red tail puffer - These fish max out at 2 inches, and are shy.
"Carinotetraodon lorteti" - Red eye puffer - Same as above
"Carinotetraodon travancoricus" - Dwarf puffer. They max out at around an inch, and are great fish, but I doubt it's what you're looking for.
"Colomesus asellus" - South american puffer. Max out at 4" and are mostly timid enough to cohabit with other creatures.
"Tetraodon cochinchinensis" - Fangs puffer. Max out at 4" also, and are quite often shy fish.
"Tetraodon Miurus" - Congo puffer. Gets up to 6inches, very aggressive, but as it is an ambush fish, won't swim around a whole lot.
"Tetraodon palembangensis" - - King kong puffer. Also an ambush fish.
"Tetraodon suvattii" - Pig nose puffer. Gets to 6 inches, but doesn't swim around a whole lot.
Those are the freshwater puffers suitable for your tank there, of which, only the last 3 really get to any size, and they are all ambush fish that won't swim around a whole lot. I imagine they would eat feeder fish though, although I wouldn't feed them feeder fish more than once a week...
For a brackish water tank here are your options -
"Tetraodon biocellatus" - Figure 8 puffer. Grows to 3 inches, and requires light brackish water. I doubt it would be big enough to eat feeders.
"Tetraodon fluviatilis" - Ceylon puffer. grows to 6 inches, and requires full marine conditions as adult.
"Tetraodon Nigroviridis" - Green spotted puffer. Same care as above.
Personally, I would reconsider having a puffer for a tank that you would want to feed feeders to, as the fish that would eat feeder fish, are all the opposite of what you want, eg, they're slow moving ambush fish that will bury and hide themselves. Coupled with the fact that it isn't in their natural diet a lot of the time, makes for a fish that doesn't really exist in my experience.
Also, I highly doubt Piranahs are legal down here in Az, the pond temps are too perfect for them.
