We have figured we have a tear but we just cannot find it. MY husband swam with goggles three days straight and even tried using a tamper type tool to tap the liner floor to see if we can create sand coming in and nothing. So weird. It's a mystery. I dumped 3 gallons of liquid shock into the pool last night out of frustration hoping to kill any living algae. I hate closing it with a tear because I know I will lose a lot of water of the winter.So sand is not coming in from your filter (obviously) and sand that course doesn't blow up over the edge of your pool so it's not coming from the surface. The only thing that's left is it's working it's way up into your pool from underneath......which indicates a tear in the liner.........that's about the only avenue left.
You are there and we are not so I would encourage you to re-examine your liner and find that tear. Unless you have sand ghosts and sand goblins, I can't see it coming from anywhere else.
Secondly, I believe you have an algae problem as well. Can you post a full set of test results and we'll help you with that.
NO it is solid concrete mix bought at a home store. It's gray.You mentioned that you have concrete inside PVC under the stairs to hold them down. I'm wondering if the concrete could be dissolving from the ends and dropping the aggregate onto the pool bottom. Does the concrete within the tubes look similar to the sand found on the bottom? Is the concrete in the tubes a "home mix" and if so was the same material that's under the pool floor used for aggregate? Maybe you could remove the stairs for multiple days and see if the deposits stop.