I've noticed on several pools, two of them are brand new, that when I add CYA to the pool, small amounts of it blow back into the pool through the return jets. How is this possible? When adding CYA, I put it into the skimmer slowly with the filter set to filter. These are sand filters with multiports. I just don't understand how fairly large granules can pass through a sand filter, when extremely small particles like algae can be caught.