Well, I took a closer look at my steps today. I tried some crushed vitamin C to see what that did to the stains. Not much. But now I believe I have it figured out.

I can see that it's a rough surface on top of the plaster rather than the plaster being pitted. I believe the stains are just scale encapsulated dirt. Does that make sense?

I used a pumice stone and it does work but it's slow and I'd go through an enormous amount of pumice. I'm going to try wet sand paper and see how that goes. Does anyone have recommendations on what would work best for this?

I'm still not sure why this happened and why it's so extensive. It's mostly on the horizontal surfaces. I know that the hardness was low when I opened. Before I closed, the hardness was low and I added a reasonable amount of powder to get the hardness within range. I may have added some after the pump was off and circulated the water with a sump pump and long hose. Is it possible that the calcium not being circulated enough could have caused a problem?

Does anyone know anything about this hydroquest 100? I love to find something that would help reduce the manual work I have ahead of my trying to sand this stuff down.

Thanks,
Lenny