It turns out that there is a small hole in the plastic piece at the bottom of the filter -- the piece into which the grids are placed. It is supposed to have a tube sticking out of it as an air release mechansim. Instead, it just allowed things to bypass the filter. I am guessing that something heavy, like CYA, was especially prone to go out this hole since it was near the bottom of the filter.

We plugged the hole until we get the replacement part (this week). There is an air release valve on the top also, so we should be safe for a short time period.

Anyway, thanks for the help.