My 3 inch RTC is now 7 inches. I have had him only a month. You will not be able to keep it in a 55 gallon for a year. Better get that 300 now! And like others have said, you'll need more later!
right now he is in a 55, and then i one year i will be getting a 300. and thats where he will live. i just dont want him to get too big before that. hence the question about slow growth