Hi all, I can't keep my pool clean. It's a brand new AG 25x15 pool with a sand filter. The water is crystal clear EXCEPT for this brown/black sediment that collects in "dead spots" on the bottom of the pool(it looks like dirty lines). When you brush this stuff it disappears in the water. It always comes back fast too. If I vac the pool . It will look like I did nothing in about 4 hrs. I have vac'ed the pool directly out to waste twice and it still comes back. I have tried raising the chlorine levels and lowering the levels. The same with the PH. On a recent trip to the supply store. I had them test my water sample and everything was OK. The only thing they thought it was, is black algae. They sold me black algae killer and that didn't work. The pool came with a cartridge filter system . We even changed to a sand filter to solve this.
WHAT IS IT? I am open to any suggestions. Including selling the pool