Does he eat the grass? I love those kinds of mini-ecosystems, where the animal can eat the plants that grow in the tank while the plants feed off the waste from the animal. It's a mini-circle-of-life!
Although it still usually needs our intervention to keep it balanced (i.e. some cleaning, watering, light, supplemental feeding, etc.) but not as much as a totally artificial environment.
Can you post another pic of your box turtles...pleeeeeaaaase.