Matt ~ we have a DE filter, not a sand filter. We started up the system on Sunday for the first time and we put 4lbs. of DE into the skimmer as recommended in the instructions. That's why I initially thought it was "overflow" DE that got pushed through filter and into the pool. Since there is a smaller amount down there now than before we vacuumed, I'm still wondering if that is what it is. I'm wondering if it will slowly disappear over the next week. I'm keeping an eye on it.