I just had a chance to try out copper sulfate and copper acetate for, as the instructions say, "immobilizing fast-moving ciliates."
Copper sulfate (one drop per drop of pond water) - indeed all ciliates on the slide were completely immobilized.
Copper acetate (apx 1/4 drop per drop of pond water) - same result. Ciliates immobilized.
The water drop feels pretty dead with all the pond favorites immobilized, but I suppose if there is a need to have ciliates completely immobilized to study an internal structure or take a photo, this is a viable method.
On an unrelated note, I may have gotten a photo radar ticket today. Darn it. if so, there goes microscope/hobby money down the drain for penance of my crime. But there's hope 'til it arrives in the mail.