here is my two cents. for a beginner, pygos always stand the chance of taking one another out, specially if conditions are not good. they are also messier, they eat more and there are more of them. you could get a serra species like a rhom. they grow slowly about 1 inch a year, so most size tanks will be fine for a long time, and you dont have to work about cannibalism. the water needs ph of 7 and a temp of 78, thats about it. but piranhas are skittish get a 75 g tank and a midas cichlid, you wont be upset