i had this same problem that i recently took care of. i knew the rtc would grow at a tremendous rate but not THAT tremendous. a 55 gal is just barely enough to house a 3 month old juvie rtc. when mine hit 7 months, he already stretched 14 inches. you have a few options. you can take the easy way out and offer your local pet shop a trade. sometimes they will give a few bucks in store credit. if your rtc is immaculate, they might offer you more. usually around 15-20 bucks. this is the route i took due to cramped quarters in our apartment. another option you have is obviously setting up a larger tank to house him in. probably the most costly option. and when i say larger, not a 125 gal. if you want to fully enjoy your rtc you're going to have to go big. at least a 220 gal. this will ensure another year to raise your cat at home. the last option i know of (a friend of mine did this) is do some searching on the internet and see if you can find a company that makes polyeurothane liquid containers. my buddy has a 400 gal container that he keeps a 2'6" rtc and a 2'3" tsn in. they're well healthy and get along great.
if you're questioning if your tank is big enough when you look at him, it probably isn't. start doing your research and figure out which way is the most cost effective to your lifestyle and make sure it's the best decision for your rtc. parting with a red tail is not easy, but i got rid of mine on friday 8/31/07 with his health and well being in mind.