Build-up by returns, don't think it's algae


Well-known member
May 31, 2016
Toms River, NJ
Hi all,
I seem to have a build-up of tan/white clumps under the returns in the pool. I don't lose any chlorine overnight, CC is 0, and it's nowhere else in the pool. When I brush, it "poofs" away and dissipates. The rest of the pool is clear and nothing is kicked up when I brush.

We replaced our sand filter this year and it has new sand. I've also added DE occasionally to get rid of the small particles (we had a lot of pollen), but backwashing should have gotten rid of the DE. I SLAMed for 2 days, passed OCLT, did a mustard algae SLAM for 24 hours the other day too in case it was that, because it had the yellow-ish tinge, but it came right back under the flow of the returns only.

Any ideas what it could be? Perhaps very fine sand particles? I'm considering just vacuuming it to waste.