Cal mag?

Do i need to use cal mag is Use tapwater that sat out and flora micro hardwater? What id i dont use flora micro hardwater? My tapwater is realy hard, and i have not got the ro machine yet. I heard to use cal mag like it wasw a must not a plus. But obly if in ro. Then last night i heard it was a good idea so i checked it and it looks good. My nut res and water skills are deplorable so i working on them. First grow dif system only ventilation was prob i thought, lol. So cal mag anybody?

you should only use or need to use cal mag if you are using RO water...tap water and you wont need to use it..