I would never feed mine any iceberg lettuce, it has very little nutrition in it. I dig out dandelions and wild lettuce whenever I find them and grow them in pots and around flower beds. I did have some Purslane for my own use but the turtles dug it up and ate every bit of it. Wild plants have far more nutrition than commercial crops. I've seen one of mine with a snail in his mouth, so of course I don't put out snail bait. Other small bugs they follow around the garden plants and watch for a while and then snatch at them.
I fed mine whatever you would feed a cat or dog or rabbit. And some stuff from the pet store, they could pick and choose.
They have a very sophisticated taste, and my late DH used to say that my turtles ate far better than any poor kid in Africa!