black particles in chlorine water test samples

Pool water looks great as usual. All water tests have been normal. The only change in my filter system is that I have not been using the SWG side of my combined Sand & SWG filter due to manufacturing issues. Instead, for the last month, I've been using the 50 lb. sand filter on FP mode (non-SWG) and bleach to maintain CL levels.

Last week, I didn't simply toss out the water in my Taylor CL / Br test cell. I just left it on the window frame (I rest it there to utilize natural light). The next day I noticed several black particles. At first I thought it might have been an insect laying eggs in the water. Curiosity had me repeat the same steps. Three successive tests have had the same results so there's something going on here. The longer I leave the tested water in the cell the darker (more particles) the tested water gets.

Has anyone had this same experience?
 
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