I really appreciate all the help. The stains look like dirt in the pool. Since we are in Az., and susceptible to sand storms, that is what we thought it was, until I realized that the cleaner was not getting it up. It is a light brown (sand) color, streaked down the sides, and mottled looking on the floor of the deeper end, looking just like dirt! It feels smooth, does not feel rough to me, and happened overnight - literally. We had backwashed, then filled and shocked and the next day it was there. It has not gotten worse over the 2 weeks it's been there, but has not gotten any better either. I had used "Stain Treat" last week, and it did nothing.
The vitamin C tabs did nothing when I tested them, confirming what the pool guy said - very surprising if it's iron. I took a water sample to another place and had everything tested. Iron was low, copper a little higher. Since we have not drained the pool in a very, very long time, the solids are way too high.
We have decided it's time to drain, and do an acid wash while it is empty.
So, that brings up more questions. What was suggested was a 3-1 ratio of water to acid, and to spray it with a chemical sprayer, then keep it lightly misted so it doesn't dry out. Then brushing it and rinsing. Is this the proper procedure?
Would you suggest something else while it's empty?