Well here are the latest updates.
Didnt get to do h2o testing this weekend.
In the last couple days, one of my tangs came down with ich. This is the first time I have had a case of ich in 8 years where I thought that there was action necessary beyond a water change and better nutrition.
It was spurred on by suddenly increased aggression from a purple tang who had been living peaceably with my blue.
The ich came in on a chevron tang, who still had a couple barely visible spots. Purple had a dusting, and the blue (although otherwise seeming very robust) is covered with the raised bumps.
Because of this I tore down the whole tang, and QT'ed all three tangs. I have never done this before.
I am wondering if vodka might have been causing stress. I pride myself on never getting ich, and that when signs appear that I can usually remedy it in DT with good husbandry tactics.
Vodka dosing may continue...or it may not. Right now I have no other explanation for the sudden change in the state of things.