Once the bleach is in the water, there is no longer a risk of explosion from mixing, so putting the cal hypo in the skimmer isn't a problem in that regard. Whether you put the cal hypo in through the skimmer or put it straight into the pool, I would dissolve it in a bucket of water first and then add it gradually--that will keep it from clumping up and obstructing your piping, or from laying on the bottom and bleaching your liner. I seldom use cal hypo in my pool, but when I do, I dissolve it in a bucket of water and then pour it slowly into the return stream so that it gets mixed well into the pool.

Janet