Here is what I got:
17.9mg CaCO3/Liter * 1L/1.0567qt * 4qt/1gal * 20gallon = 1354.8 mg CaCO3
1354.8mg CaCO3 * 1mol CaCO3/100.0g CaCO3 * 1mol CaCl2*2H2O/1mol CaCO3 = 13.54mmol CaCl2*2H2O
13.54mmol CaCl2*2H2O * 147.0g CaCl2*2H2O/1mol CaCl2*2H2O *1g/1000mg = 1.99g CaCl2*2H2O to raise 20 gallons of water by 1dGH.
For the mansesium, the last two lines need to change:
1354.8mg CaCO3 * 1mol CaCO3/100.0g CaCO3 * 1mol MgSO4*7H2O/1mol CaCO3 = 13.54mmol MgSO4*7H2O
13.54mmol MgSO4*7H2O * 246.4g MgSO4*7H2O/1mol * 1g/1000mg = 3.34g MgSO4*7H2O to raise 20 gallons of water by 1dGH.
This makes more sense than the Fertilator numbers quoted by Bharada since the calcium chloride dihydrate is 27% calcium, while epsom salt is only 9.8% Mg - it should take a lot more epsom salts than calcium chloride dihydrate to raise the GH by the same amount.
72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.