Due to sanitizing purposes I do not use any soil-type substrates. For temporary & QT enclosures I use paper towels. For longer term I use repti-carpet and have plenty of objects for the frogs to sit on so they are not in constant contact with said carpet. It is easy to pop in a clean piece while the other piece is being cleaned, so that's nice. I have also used BPA free white or opaque cutting matts for the bottom of tanks. They aren't as rough as the repti-carpet, but not as slick as a plain tank floor, and are even easier to clean and sanitize than repti-carpet is.