Being dark brown isn't a bad thing or an indication that it's dehydrated, they can change colour. Mine is dark brown during the day most of the time and some shade of green after the lights go out.

20 quarts is kinda small. Real live plants can help keep the humidity up. Does he ever use the water bowls?


If you haven't already read it, here's a care sheet for gray treefrogs, you can house your American Green in a similar way:

Frog Forum - Gray Tree Frog Care and Breeding