You will want to add the Calcium Chloride to the change water before adding it to the aquarium, and it should already be added to the aquarium water if you need it. The goal is to keep the buffering agent the same. Don't use it to rescue a crashing system (too late), but use it as a preventative measure. You may find your tap water already is "hard" and don't need to do a thing. Test your tap water for hardness, and if it is rated "moderately hard" you are good to go. If soft, you will need to add CaCL.
Note that I was using CaCL when I was using rainwater for aquarium changes. Before I got my well-water treatment system I could not figure our why my fish went nuts during water changes. Turns out they did not like the natural gas in the water (probably tough to breath that stuff). I added CaCL to the rainwater since you cannot get much softer than that. When I tried adding CaCL to my well water, I found it forced the natural gas out so I was able to back to tap water. Now my water treatment has a de-gasser built in.
My PH from my well is around 8.0, and will vary between 7.9 and 8.2. Stinky for most aquarium plants, but my fish love it.
I did not know that nitrogen fixing bacteria retire under 50F, I learn something new every day.
I would like to understand what you are doing with a filter better, tube socks... that's a new one! I hope your fish like cheese.
