A 30 gallon is a little small for a piranha. You could put in a lone Red Belly but they do better in at least groups of 3. Also exodons should be kept in at least a 55 gallon since they are very active fish.
I would recommend Dwarf Puffers or a mating pair of convict cichlids. If your willing to give brackish a try then I would suggest a Figure Eight Puffer. If it was up to me though I would go with a bunch of cardinal tetras, otos, and some other small community fish of your choosing. I currently have Cardinals and they are stunning. I actually have that tank next to my red belly piranha tank and everyone always seems to be more interested in the cardinals over the piranhas.
Also be warned puffers are not the easiest fish to keep and require a lot of care. So if you decide to go with puffers do your reading before buying. I have kept Indian Dwarf Puffers in the past and they are a nightmare to take care of especially if you need to go away on vacation since they don't accept any flake foods or pellets. On the other hand Convicts are great fish and are quite aggressive which seems right up your alley. Also if you get them to breed you can always bring the babies to your lfs and exchange them for credit to be used for food and supplies. FYI breeding convicts is not very difficult so if you think that would be something you would like to do go for it.
