you can add sand to an existing tank by pouring it into a pvc pipe that you have submerged down to the sand bed. the larger diameter the better. just cut the pipe so that when one end is at the top of the sandbed, the other end is slightly out of the water at the top of the tank. carefully pour the sand into the pipe as you move the submerged end where you want the sand to end up. if you are taking the sand from an existing tank i would suggest rinsing it well with some old tank water to get as much junk out of it as possible.