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 from their messy eating habits. 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". It feeds primarily on the scales and fins of other fish.
"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 but considered more aggressive.
"Carinotetraodon travancoricus" - Dwarf puffer. They max out at around an inch, and my pinned topic on them is at the top of this forum.
"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 fish available.
For a brackish water tank here are your options -
"Tetraodon biocellatus" - Figure 8 puffer. Grows to 3 inches, and requires light brackish water.
"Tetraodon fluviatilis" - Ceylon puffer. grows to 6 inches, and requires full marine conditions as adult.
"Tetraodon Nigroviridis" - Green spotted puffer. Same care as above.