In it's natural environment, a cockatiel will feed from the ground most of the time, this is where they find grass seeds which make up the majority of their diet. Ensure that your cockatiel has easy access to the ground of their cage. Some pet stores sell metal grills that cover the bottom of the cage to aid in cleaning but these are not really recommended for a cockatiel.
Most sands contain small pieces of eggshells and mussel that cockatiels eat from the ground to help with the bird's mineral metabolism. You can get gravel paper, which is just paper with sand and seeds glued to it, this type of floor covering will not satisfy the cockatiels natural desire to pick at and scratch the ground. This type of covering also makes the bottom of the cage very hard, so if the bird was to fall off it's perch it could hurt itself.
Bird sand is always recommended as being better than gravel paper, so try to get it if you can.