It's been a years long process, but I think I have finally identified the remaining sources of metals in my water. Up until this year I have been dealing with metal stains and have had to do one or two ascorbic acid treatments each year. This year there have been no metals and no stains. Hopefully this information might help somebody else.
Several years ago we had our pool refurbished, including a poured concrete coping. The concrete was colored brown, and the contractor put a brown powder on the wet concrete before stamping the pattern. Once the concrete dried they put on a coat of clear sealer. Looking back, however, they were rushing when applying the sealer due to the threat of rain.
Last fall I was talking with tech support at CuLator, and the support tech mentioned that he had encountered metals leaching out of colored concrete copings. I confirmed that the concrete colorant and colored powder do contain metals. I applied a coat of sealer last fall, and I think that this is the biggest reason that I have been metal free.
I have since learned that it is best practice to put on a new coat of sealer each year, and that it is important to match the type or brand of sealer year to year.
The other probable source of metal was the chrome trim ring on my Pentair Amerlite pool light.
I replaced the trim ring several years ago, but within a year the ring had turned a tarnished black. I checked my bonding wires, but everything seemed fine. I put a zinc on the light, but the zinc dissolved within a year.
This spring during my travels around the Internet I discovered that the replacement screw from Pentair for fastening the trim ring to the niche was brass. The screw that I had was stainless steel. Pentair tech support couldn't explain the change, but did confirm that they now recommended using the brass screw. A quick web search showed that stainless steel has a much higher resistance to electrical current than brass. I am assuming that the high resistance of the stainless steel screw was causing galvanic corrosion between the trim ring and the (bonded) niche, resulting in metal being deposited into the water.
I purchased a new trim ring, which came with a brass screw. So far it is bright and shiny.
I hope this information is useful to somebody.