Thanks, Donaldson. FWIW, this was first noticed after a fresh fill, and with the ionizer disabled. I have not run the CH test in a lttle while though - since it pretty much stays put I only test CH after fills or partial water replacements.
Previously, I had noticed the copper concentration would keep creeping up, even with the ionizer on its lowest setting, so at some point I just unplugged it. It had not yet been reactivated on this fill. I had previously seen interference more like a fading endpoint, which I had assumed was Cu interference, but without this coagulation visible.
I will try a test on straight fill water today I think and see if it does the same thing?
Re. copper: I think I inderstand the objections here (masking poor sanitation, staining potential at higher concentrations if pH not carefully controlled, test interferences). If my wood tub were lined and I could tolerate the higher standing FC concentrations advocated for TFP spas I would just dispense with it. I have been trying to learn as much as I can from TFP methods, which seem worked out and very effective, and work out a effective method tailored for my unlined tub (let Cu/Ag at reasonable concentration keep algae down, sanitize after each use with dichlor/bleach for the other bugs, oxidize weekly to clear waste, and run a much lower standing FC concentration stabilized with some CYA). It sure would be easier with straight TFP, but I think it would pulp my tub if I played strictly “by the book”!