Deionised water replaces hard water?

I agree with Papa_T, RO, DI, D water can all harm your probes. TDS/PH, they are a purfied version and have little to none of any mnerals so they are very aggressive and strip ions off things to make it have a equilibrium again. This is why all RO tubes are plastic as they would destroy the metal pipes. Its expensive to buy, and wasteful to use on growing I find unless your dealing with bog plans that need very clean water. Better to collect rain water for a easy cheep option.

You can’t PH straight RO or deionized water. Your PH meter will aimlessly drift as there’s nothing in the water to charge and get a reading. Using a PH meter with water of 0 PPM can actually damage your device. If you have done this I would recalibrate your PH meter. Now if you use RO or deionized water and add things to it then yes you could PH it properly once you add in some TDS into it.