Can i keep one red belly piranha in a 20 gallon tank?

i know most people are going to say they need to be kept in a group because they are supposed to get along with there own kind but nothing else well i had 5 of them they were all about 1 inch but one of them attacked all of them and now i am just down to that one... despite his small size he is really mean and when i go to feed him he like trys to attack the glass... i have a 55 gallon but i currently have a pair of jack dempseys in there and i am not going to mix the piranha with them but i dont know what to do... and also people also told me that 1 piranha by himself is okay... but if the piranha will get too big for the 20 gallon which my local fish store said that 1 piranha is fine for a 20 gallon but im guessing he could have said that just so i would buy the fish... so what should i do? i am not about to give any of them back..... thanks for any help