I've been giving Paki leaf lettuce. At the pet store, I think they were giving him iceberg lettuce. At least, it was a much lighter green. He likes the leaf lettuce and will usually eat all that I give him.
I have done web searches to find out what else I can try. One site says raddichio is a no no; another, says it's okay. The same is true for herbs, kale, spinach, and many other vegetables.
Is there a web site that lists vegetables that can be given on a daily basis that you all trust?
http://www.beautifuldragons.com/Nutrition.html
ignore the kale thing it a great staple feeder
If you scroll to the bottom of the page there is plants etc that you can look to see what can be put in the tank or fed