Do you have any pictures? And IMO a 55 gallon is a 55 gallon..and is way too small for piranhas...lol they are extremely fast growers and soon enough you will see that the 55 looks like a glass of water with them in it.
They are going to fight if you leave 6 rbps in a 55 gallon.
i no that lol i have a bigger tank for them but they are only 4 inch so if i put 6 4 inchers in a 240 gallon tank with two breeding 8 inch piranhas they will die so im keeping them in the 55 till they are 5 5.5 inch to give them a better chance