I had two ADFs and did some reading up on the subject. You can leave the bottom clean and empty of substrate, sand is good, gravel is OK, but you want to be sure that the gravel is not small enough that the frog may get it in his mouth.
I've seen photos of ADFs with small gravel stuck in their mouth, and then you have to try to get it out, or gravel small enough that they swallow it and it causes an intestinal obstruction.
I had some gravel, at one point sand, and at another time bare bottom. I would go with either a bare bottom, or a heavy, large grain sand, like pool filter sand.
Even if you go bare bottom that's fine, in fact you can really see debris and keep the tank clean, you still need to have some neat places for them to hide, and either fake or real plants.
If you get fake plants, make sure you get large leaf type, silk is really good. You don't want to get the plastic plants that have sharp pointy leaves, because they can injure their delicate skin.
I hope this helps... ADFs are just adorable.