You just never know what was going on in the tank previously. It may have had some sort of bacteia that may still be present especially if the sand is still damp.
I would add a mix of bleach around 10% and let it stand for a while then give it a really good rinse and let it stand overnight filled with water just to be on the safe side.
When I first set up a tank I bought a bag of sand from Evans Toys that is for children's playground use.