It's just alkalinity in one and calcium in the other, standard type of products. They market it to nano reefers. Ounce for ounce it's probably a little more expensive than some of the other brands. I used this when I had a nano tank. I have a larger tank now so I get my alkalinity and calcium along with magnesium from BRS.
You do not need to turn off your skimmer when using any of this type of product. You want good water circulation when adding these and the skimmer actually helps with this somewhat.