niger might be ok but it really depends on the piranaha, my old lax coach had 3 RBP and a channel cat in a tank together and they lived together for 3 years until one night the channel killed all 3 of them. but on the other hand i had a channel and a RBP together and the RBP ate him in 5 mins.... tore him completely in half. so whatveer fish you chose, theres always the chance your gunna lose it