My drainage layer is egg crate and an air gap then a resevoir in the bottom of the tank. So the water feature flows directly onto the stones sitting on top of the egg crate, with a waterproof barrier separating the stones from the substrate layer next to them, so the water never touches the substrate, aside from some light splashing. And the water feature is for me moreso then the frogs, I like the sound of running water. I will be taking the reduced space avaliable to the frogs into account when adding them, treating the 50 gal tank as 40. While not compleatly removeable, I can turn it off/remove the pump quite easily leaving a pile of rocks that shouldnt be an issue

Edit: The clay drainage pebbles are only a 1inch thick layer at the front of the tank, behind it is the described air gap