For what it is worth...
I did a water change, stopped using Excel and switched to using Flourish for my traces rather than CSM+B. And the net result is that my shrimp appear to be much, much, happier. Where they were lying on their side and kicking in spasms, now they are wandering around grazing on detritus as they should be.
Now, it's possible that the water change removed some other contaminant. For instance, nitrates. But NO3 in the tank now is high now (50 ppm) too.
If I were more willing to sacrifice japonicas to science, I would dose with Excel and watch their reaction, water change, then dose with CSM+B and watch again. But I am getting good growth with ~30ppm CO2 and Flourish, and my wife and kids adore the shrimp, so I will not bother.
By the way, since Tom Barr has posted here, I will mention that I've gone over to EI (with Flourish) instead of PMDD (with CSM+B). But NO3 seems high at the end of the week (50 ppm) while PO4 seems right (1 ppm). Should I cut back on the NO3 or just not worry about it in the spirit of the EI method?