[ray-the-pilot]

CaCO3 will increase KH and GH but it is practically insoluble in water. You will only get a chalky gunk adding it to your tank. The CO2 in your water will eventually dissolve the mess but it will linger some time in your substrate causing erratic results in your CO2 levels. Also, adding CaCO3 doesn’t address the need for Mg+2 another element in high GH water.

Actually, I recommend using a commercial RO water reconstituting formula or adding some of your tap water back into your RO water to get the right GH/KH.

BTW If you do have a low GH/KH tank, I recommend keeping CO2 levels below 20 ppm. At this level you will still have rapid plant growth and pearling of all your plants.

BTW2 Since you are using RO water, the pH/KH/CO2 relationship will work perfectly for you.