How old is it?
I honestly would keep it in a smaller enclosure to get some initial weight on the little guy. I'm sure he appreciates the space in a 20g, but tge amount of flies you would have to dump in there to make sure he gets enough to eat would be extreme. I had four cobalt tinctorius' in a ten gal grow out viv and it was too hard to keep weight on them, let alone gain weight. I ended up pairing them up and moving them into 128oz. Containers. In two weeks they grew more than the month that I had them in the ten gallon. Just my experience with little froglets. Auratus are awesome frogs, I hope he thrives!