I am the newly appointed pool boy for my father-in-law's new (old but new to him) pool. The pool was not maintained but was covered for a couple of years. I'm not sure but I think the filter was in operation at this time. The water was pretty good when the cover was removed. In any case, I know little about sand filters other than the very basics. I have a neighbor with an AG and he has taught me a few things but does't use BBB. He used playsand when he replaced his filter - which I think I read was a no-no. I would like more info on what the process is to determine when you need to add or change the sand.