Has anyone done something similar? I am curious about the ratio to
use or if other chemicals should be used. I plan to use 1/3 of each
and then apply a measured amount of the end result to a gallon of
water and measure the rise in GH/KH and use that to develop a dosing
plan to apply when I change water.


GH-
MgSO4 and CaCl2 work very good and dissolve fast and have no effect on
the KH.
KH-
Baking soda for the KH.

CaCO3 is rather slow to dissolve, powdered aragonite is a bit better
but you are adding both KH(CO3 part) and Ca there.

CaCl2 is sold as pool hardness increaser. I got a 7 kg tub for 6$.

Regards,
Tom Barr