Seachem Excel (CO2 stuff) contains glutaraldehyde, which is definitely not good for shrimp. I really can't say how sensitive shrimp are to this stuff -- you can easily find more info on this on the internet. Perhaps light doses are fine.

My shrimp seem to go into a let's-congregate-in-the-corner-and-not-eat funk when my nitrates get to around 50ppm or above. Within 2 minutes of completing a 50% water change, they are back into action, eating again. I cannot say for sure that the nitrates are causing this, but my WCs fix something. This has not happened to me in months, mostly because I cut down on KNO3 dosing, substituting K2SO4.