Ok here is a brief history. Last year I was battling a mystery "dust" that would settle on the bottom of the pool. The rason I called it a dust was because it was extremely fine and would 'blow' away if you wave your hand at it. all of my levels were good and could never figure it out. This year has been great... until a few days ago. The same stuff showed up again. My CYA is high at 70-80 but I have been compensating by keeping my FC at 8-9. I do keep the solar cover on when not in use and sometimes that is for several days. I am wondering if that has anything to do with it?? I vac with my robot every time I roll up the cover.
As a side note we also had a couple of good storms over last weekend. At any rate I had notice the substance on Monday and my FC was still at 9. I ran the vac and added about 1/2 bottle of 8.25% bleach. Covered the pool that afternoon. Bought more bleach. I haven't touched the pool til this morning (Thursday).I rolled the cover back and yet again there is the dusty substance on the floor. I check the FC and it is still 16.5! I added another Gallon of 8.25% this morning and started the vac. This should put me at the 23ppm FC mark. I will check the FC again when the sun goes down and see where I am at. Then will do the overnite test to see if it drops from that point.
Note #2: I suspect the the overnight test will be fine. I did this same exercise last season and the over night test yielded little chlorine loss as in .5ppm.
Ideas, questions, concerns??? I am at a loss on this one.