Any lettus can do. I perfer getting special turtle food from a pet shop.
Well, the most commonly used one starves them to death. Here's some of the best resources. They take a lot of VERY specific care and not providing it results in death. I'm not sure how old, but approximately 95% to 98% of these long lived amazing creatures in captivity purchased as pets die before they reach one year due to improper care. Tragic.