The Substrate is of minor importance in a Paro-tank. It’s not a tank for gardening. There are successful breeders using no subtrate whatsoever, others (as myself f.i.) use calcium-free gravels or peat. Deepin’ peats suggestion to use dry beech or oak leaves is very good since it come near to the natural situation in the rainforest peat swamps of south-east Asia.
A congratulation for deepin’ peats replies to many questions in these forums; they are strictly to the point and exhibit knowledge and expertise.