it Is very hard water, I use rodi in my aquariums. I have several planted caridina shrimp tanks, it’s pretty much necessary to use remineralized rodi water as a high KH is bad for their molts. For my plant farm and ‘aquascaped‘ tanks I use tap with top offs with pure (TDS 0 ) rodi water to combat evaporation during the week, and do weekly 50% water changes. My fish don’t seem to mind, and parameters are stable.
Aquarium ”Soils” depending on their composition will buffer the ph of an aquarium down. Some less than others, some more, all have a capacity that will diminish over time. But that depends on other aquarium parameters such as the amount of plants, mineral content of your water, and stones. aqua soils don’t last very long in our water, they still work, but overall it doesn’t have the same ‘nutritional’ balance as day one. RODI has worked for me, i have not used water softener pillows. I have tried aquarium products to buffer down the ph and things like that, but that’s more of a headache involving a more hands on approach with testing. With Rodi and remineralization i test once for what i want my parameters to be, and I have the measurements of my remineralizer ready for the quanity of water im prepping. Doing the same for tap would require testing before and adjusting each time because of the inconsistencies in tap water quality especially after heavy rain.