you can add to it, but dont add a lot at a time. After the tank has been established, the sand will be live by then. When you put new sand over the top, you can kill off some of the bacteria in the sand if you put much more than like an inch i believe it is. Then you wait about a month or so and add another inch i think. I cant remember exactly how it was, but yes, it does matter. You just have to do it slowly.