So I've read through your other threads about this issue, and I think the consensus is metal, most likely iron or copper, and I know that the pool store has tested and shown very little if anything. Jack's has a metal stain id kit I believe. Taylor sells Iron and Copper test kits, and I've used them and they are easy to use. You could try purchasing those. The thing is if the metals are depositing in the plaster, they won't show up well on tests.
Your other option is the AA treatment which is a PITA and I'm not sure that's something that will be worthwhile, because even if it does work, you're looking at sequesterants going forward. Long term, monthly maintenance treatments.
As for the supply water, they trucked it in initially, from where? Are you confident that water was metal free? And since then you've been topping off or draining from rain, which has diluted the copper that was added the first time the pool company gave you crappy advice.
Still since you are getting reaction from a vitamin c tablet, that means metals.
Now, I don't know where Leonardtown is, but "city water" in many places can be just a municipal well. For example, here in my town our water comes from Lake Michigan, but the town just west of me is a municipal well and the water is gawd aweful.
I would go for trying the ProTeam metal magic and see if it works, and it may be something you'll use maintenance dosing for.
