It will be difficult to control: The resin will remove hard water ions until it is completely used up or until there is no hardness left in the water. So if you want zero hardness, use as much resin as possible. If you want "low" hardness, then this would be difficult to control.
Also, cheaper resins replace the hardness ions with two sodium ions. Sodium ions can then build up in the tank and might demonstrate some toxicity to the plants (not sure at what level).
Better to mix your tap with RO/DI water to get the desired hardness.
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.