Awww...Look at Igor and Izzy! Such cuties!!
Annibale, here are some of the things I feed Yertle, my turtle: lettuce, tomato, strawberries(he adores them), spinach, tuna fish, dried shrimp treats from the pet store, sardines, grapes(remove seeds), hard-boiled egg yolk(he goes crazy for those, too), etc.
My mom just got a little turtle, Lilly. She said that Lilly absoulutely LOVES steamed cauliflower(cooled down, of course). I'll have to try that with Yertle. Oh, don't be alarmed if he doesn't eat every day. Yertle will sometimes eat every day for a week or two, then he won't eat at all for a few days, sometimes even a week.
They also need access to clean water at all times. I let Yertle roam the apartment, so I have a little "swimming pool" for him with a ramp so he can go for a dip whenever he pleases.
Good luck with little Herman. Turtles really are lovely creatures. They're just so gentle and beautiful.