I have kept my rhom (black piranha) for just over 16 years now. Bought him July of '99. You can see the color changing on him and he has HITH that I can do nothing about...
I bought him around 3in or so and he is between 10-11in. He has only grown about 1/2in a year. Unfortunately I did not do the best maintenance over the years as 16 years ago things were done a bit differently. I used to do only a 20% water change every 4 weeks for most of this life. Starting 3 years ago I started doing about 40% every week. He has been in his own 75g for most of his life and recently got his own 180g. Lots of things I wish I could go back and do to maybe get better growth but also prevent the HITH he has. It is impossible to get rid of at this age and the stage it is.You have to post pictures of the rhom! How big is it? What size was it when you bought it?