I have one that i have had for about 3 years, he was about 8". I did not feed him nearly enough food when i first got him. I would only feed him about 5 nightcrawlers a week.
He only grew about 5" in 2 years. In the past year i have been really feeding him like i should, and he is just a hair over 24" now. I would say that nightcrawlers, mussles, and shrimp would be a great diet. Mine can eat about 5-10 medium sized frozen shrimp a day now...they ARE.......MONSTERS!!!