I have an inground vinyl liner pool, about 20k gallons. It has a Hayward DE filter, clear water, and chlorine and PH look good. The strange thing is that I'm getting sand at the bottom of the pool. It's tan/brown, gritty and granular, so it's def not algae. I've cleaned it out a couple of times, but it comes back in a few days. Any idea where it's coming from?