My first fish was a rtc. I got him at 4" in a 55 gal uncycled aquarium. Every fish died during the cycle but the rtc and an oscar. I grew the rtc out to about 18" in the 55 gal, then I bought a 125 gal. I grew him to about 28" in there before he died of brown blood desiese. I had him 2 years. Much shorter life span than what they can live(20+ years). If only there was MFK back then lol. I might still have him, and he would be a monster at 12 years old. I didn't know about ponds and stuff like that back then, but you have us here at MFK to help you out. So take in all the advice that you are getting here, and apply it fast and continous. I think frequent water changes are key in keeping big cats. They produce enormous amounts of waste, and toxins build up in their systems over time. Needless to say the cleaner your water is the healthier your cat will be and the longer it will live. My rtc had daily %25 water changes when he was in a 10 gal, at 6" I moved him to a 75 gal with every other day %25 water changes, and at 12" I moved him to a 1100 gal indoor heated pond, and he gets %40 water changes every 5 days. After 4 1/2 months he's 14 1/2", Good luck