You're probably right, Watermom. My understanding of cyanuric acid, an organic compound, is that it's not very soluble in water and while the granules or flakes do break down into something too small to be seen, they may still be of a size that gets trapped by the filter. The constant abrasion against the filter medium eventually does break those small particles down and pushes them back out into the pool, and that may take up to a week.