ScaleTec or StartupTec are good things to add on new plaster startups. They are primarily sequestrants which work well at inhibiting scale formation and issues with metals in the water... Calcium is a metal after all
Yep, not blaming anyone, its my fault, was just more focused on all the other thing tests, etc, etc. Perhaps i will just raise the water level again rather than trying to mess with removing the ring. Will pick up a calcium eraser tomorrow and test it out. If that does not work i am raising the water level and will let the correct chemisty remove the ring.
Just my 2 cents here....I would get my face down there on a tile and see could it be 'etched' into the tile? If so raise up water level to that line, you mentioned when wet does not show.
Sulfamic acid.