Welcome to the forum! Josh has given you great advice thus far! Your 18x18x18 tank can house a wide variety of both terrestrial and arboreal frogs. I dont know much about your native species but one example is the pacific chorus frog that like to be both on land and in the trees. I, of course, want YOU to pick your frog and to not let my suggestions influence you. We are all willing to help so ask any questions you have, even if they may seem odd. Better safe than sorry! Good luck and keep us posted!