I normally hate when people post questions and are given grim responses because the general assumption is that more times than not, people don't know how to properly care for their fish. That said however, a piranha really is a very difficult choice. They get large, they're extremely aggressive, and because of these things: the only realistic tankmates would need to be monsters. Meaning you cannot do anything in a 37g setup.
In all honesty, because there's so little you can do with piranhas, I would agree with every post above. Go with something else: afterall we can all offer a ton of options for things you may not even realize are freshwater options.