unless you caught it out of the ocean, the wild fish is freshwater. so are the piranhas. if they were marine they'd be dead already. you really shouldn't have gotten any fish yet, and now you're going to have some work ahead of you. first of all, wild fish, depending on where you live, are coldwater, and won't be very healthy in a tropical tank. also, wild fish often carry diseases that pet fish are not immune to, and pet fish have some diseases that wild fish can't deal with. too late now, most places have laws against rereleasing any fish that has been kept in the home. the diseases it catches can spread to other wild fish and cause a lot of problems in the ecosystem.
i don't know if you've read the thread on cycling, but you should have picked up some test kits for ammonia, nitrite, and nitrate. you'll need to test your water parameters for those three things, and do daily water changes to keep the water safe for your fish. ammonia and nitrite should be below .25 every day.