Note that putting it into the skimmer directly so that it gets caught in the filter is convenient but does not dissolve as quickly since the flow rate in the filter can be rather slow. The flow rate through sand filters is no more than 30" per minute, in DE filters it's around 2.4" per minute, and in cartridge filters it's around 0.4" per minute. Compare that to a skimmer 10" in diameter (78.5 sq. in. area) at 15 GPM (3465 cubic inches per minute) which flows at 44" per minute. So perhaps putting CYA into the skimmer to a sand filter may work OK, but it will take much longer to dissolve in a DE filter and especially a cartridge filter.