how fast should I expect my Juvenile piranha to grow ? They are all about 2 inches right now . I would like to know how fast they are going to grow per month. I have decided to keep 5 red belly Piranha in a 55 for life because 8 seems to much and would be expensive. Do you think 5 is an okay number and would 6 be a better number for a shoal ? ( remember the growth question ) .