dzb is right a 75 would house a solo serra family piranha for a very long time. Serra species "nabbig" have very very slow growth rates past the initial 1-2 year 5" mark. I keep my fastly maturing serra rhombeous xingu formation in a 30 long, it provides him 36" tank, I do not think it will be ok once he hits the 4-5" mark but since he is barely 3 1/2 I dont think we have a problem. I also got a deal on him a lfs didnt know what they had... hope this helps... I may be new here, but I am certainly not new to piranhas...