Total hardness is GH. It is pretty much the sum total of Ca+2, Mg+2, Fe+3, and Mn+2 ions represented as a CaCO3 equivalent. 20ppm CaCO3 is just over 1 degree GH (17.9ppm = 1dGH) which is very soft water. For a high light tank, this might mean you would need to add some extra Mg (via Epsom salts) and Ca (via a calcium salt - there are several to choose from) along with an iron-containing trace mix. It will also probably limit snail shell formation and growth - I have 50ppm and my snail shells are all slightly eroded.
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.