Why did your CaRx need adjusting while you were away? I have never had to have my tank sitter touch my CaRx while I am gone.
In any case, if you change water, then just make your own. If you do not change water, then you might need some with some strontium, magnesium, etc. in it already, but you can overdose on these since you cannot control the input separately.
The huge bag of baking soda at Sams or Costco is about $8. Epsom Salts are cheap at Walgreens or CVS. 50 lb of DowFlake from a Tennis Supply shop or nearly any hardware store in a cold region is about $20. This can last for years and years. You can get some strontium chloride if you want. Dr. Holmes-Farley has a great tutorial as posted above.