Sand, unless it is heat sterilized, can hold a wide variety of bacteria and fungus. It can also be very dangerous if you feed your snake on it (swallowing sand is a big NO NO). Sand is also very difficult to keep clean. Fish tank gravel can also be very difficult to keep clean. If you want to have a small part of your snake's tank covered in this, fine IMO, as long as you completely clean/sterilize it or replace it when dirty. Also watch feeding on it (swallowing a stone is also a bid NO NO). Aspen or other made for reptiles bedding material is fine, but again, don't feed on it since ANYTHING foreign being swallowed is a big NO NO. My personal favorite material, due to the number of snake tanks I have, ease of use and cost, is just plain newspaper. Not as fancy to look at, but suits me fine. Bark, depending on what kind and how clean/sterile it is may be okay to help decorate an otherwise plain tank.