Agreed, you won't be successful getting him to eat any significant quantity of lettuce when you are feeding in a separate tank. Just attach a few leafs to a clip in his main tank and let him snack on them throughout the day. It's like having live plants in his environment without the frustration of watching him destroy any live plants you put in there.
BTW, the only veggie clips I have found in the local pet store are the ones shaped like a little fish, and that one will break very quickly. I have had good luck with the ocean nutrition ones I got off of amazon. (search for seaweed clip) I prefer the wide mouth to be able to hold more than one leaf of lettuce.http://www.amazon.com/s/ref=nb_sb_ss_c_ ... Caps%2C210