One of the basic things folks need to understand when considering an RTC is that if you buy one you are most likely going to own the fish until the day it dies. Whether that's in 20+ years at over three feet, in a year or two when the fish reaches eating size, or in six months when the filtration on the 55g tank and the weekly 25% water change aren't good enough anymore so the fish gets sick and dies. The demand for RTCs over 12" is nearly zero and if your tank is over 100g very few who would take the fish for free are going to provide a better home.