Also, firebelly toads like slightly acidic soils. Mixing 50/50 spagnum and common yard top-soil will provide up to 7 years of lower PH. Most water is about 7 PH, they prefer 5.8 - 6.2, so do most mosses. (Pine sap provides the lower PH in nature, and sugars.) The down side is that acidic soils tend to get that unwanted BOG odor unless waterd and drained monthly... more like saturated. You can find sphagnum locally on the west side of tall objects. Hills, groups of trees, houses, buildings, barns, etc. Since there are no roots, you can pull whole sheets without effort. You may also find fir mosses and clump mosses on the north sides (Mostly shaded area.) of those same structures.
Mixing 33% cocoa fiber (Bed-A-Beast) and 33% sphagnum and 33% Top-Soil will make a light tank with high volume. Also, replace stone with fired clay pellets or stone-like plastic pellets. (Mix by volume not wt.)
Rinse and reuse the substrate, don't continually replace it. Rinse with untreated tap water, squeeze dry, allow it to bake in noon sun for one - two days, turn by hand two - three times a day until mostly dry, saturate in stream or river water, squeeze dry, and return the substrate to the tank. (Once - four times a year.) Replace any lost volume with a similar 33/33/33 mix, durring the, "saturate in stream or river water", phase.