what are the dimentions of your 66?, it's very unlikely you could keep a rhombeus in the tank for life. in captivity they rarley reach over 18", and that takes many many many many years, they grow on average about 1-2" a year. with a rhom the size you'd most likely be able to house it in a 66 for a LONG time. but eventually if you kept it< (unlike many hobbists who get bored of them because they won't grow) then you could upgrade to a 75, and then mabey a 125- 6'x18" footprint, if it got over 14". most people get bored and then just decide to buy a larger rhombeus.