How does keeping the water work? Doesnt it get..gross i gues you could say, do you have a filtration system of some sort?
Only Julia can answer that but I'm guessing she does, or at least she does a lot of water changes. Otherwise the water would get too dirty.

heat pad:
Would it be ok to put on the back of the tank? would the heat still come thru the background? i was thinking of getting one of the cork backgrounds i guess or which one would you reccomend?
Yes you can put it on the back, and as a matter of fact it's the best option for tree frogs, because the heat you get everywhere. Cork backgrounds are cool and you can fix all sort of thing in them

substrate:
Im worried about cocoa husks only because if its swalowed...hearing the horror stories because im a bearded dragon owner. :/ its safe for whites? i hear they sometimes use their "hands" to push food in their mouths. Ya i was planning to get the biger flat river rocks..like 1-2inches..not the small pebble looking ones..just for that reason to avoid impaction. Which plants are great for whites? Im worried ill put the wrong ones in.
I feed my when they are on the top of the tank. That way they don't swallow anything but the food.
You can use big plants which reach the top of the tank with strong leaves that can hold the weight of the frogs.

Hope it helps