I have a beautiful Pastel Pen. Cooter, I got him when he was about 3 and 1/2 inches, now he's about 4inches so i'm guessing he's about 2 year old. I'm wondering the grwoth rate of this guy, I know they can get quite large but I don't know how fast, need help so i can prepare a larger tank if he needs one (currently in a 60 Gallon)