I used to have a 36" in my 750 indoor pool / pond. He would have half a head of lettuce every day. Every once in a while I would give him a couple apples cut in half. He was a big boy! If you have the space get one, they are amazing animals, if you dont have the space, dont bother.
I used to have a smaller one around 5-6". Poor me, I listened to the Petco guy and had no idea how big they get, not to mention how FAST they grow. I had to give him away because he outgrew TWO tanks in about 5 months, it sucked.
Anyway, At around 5-6" they'll readily eat anything. I had mines on pellets, algae waffers, nuts, all kinds of fruits and veggies, and cocktail shrimp.
But yeah, as mentioned before, THEY GET HUGE. I'm surprised they're so common at pet stores.