It's good to hear they have started to eat. How have they been the past few days? Hopefully they were just stressed and have settled in and are eating more now. Have you added any more hiding places to help even more with that?

I have no experience with lava rock, in my tank I use smallish (about half an inch or so) river rock. The person who owned my frogs before me kept them on gravel with it sloped up to form land and when I transferred them over to the tank I was keeping them in and cleaned their old tank it was FULL of grossness. So I just have enough to cover the bottom of the tank and use flat rocks on top of each other to act as steps then the area above the water. So far that seems to be working to stop their poo from hiding and building up.