We're not saying you can't use sand but we are just trying to warn you that if the sand gets sucked in the filter, then the damage on the filter's impellor is likely to happen. Just make sure the sand won't really try to cloud everything which will eventually go inside the filter.
If you try to fill in the new water to the tank, use your hand or a plate to smoothen the flow of the water. This also helps not to disrupt the decors.