Deionised water replaces hard water?

I have the same problem.mine is 8.0 so not nearly as bad. i just use a water filter. with my ppm meter, I dont get a reading at all, so less than 100 ppm after filtering it. I think its 500 ppm before filtering i use the boggie brew filter. I ph down most of the time with my cal-mag, but if not feeding anything I use my phosphoric acid and you will need something stronger like that with such basic water

A different way to go COULD be ( and I am ASKING here, as I had the thought but am not sure) Buying bottled spring water. I tested some of them from the supermarkets in France and they are pretty good, PH6.8,

My Opinion, I am not a phD or Professional grower, so it is what it is. Deionized is (sorry German here) as far as I can tell basically distilled water or? basically that is water with 0 PPM. This usually will have a PH of 7.0 (anything else indicates that it is not very pure, so I'd leave that one brand alone. Basically this is much the same as using a Reverse Osmosis Filter (which I would recommend / prefer). Short answer is: yes you could, but make sure to add your foods and remember that in most water you have trace elements like Iron etc. In Deionized water you do not (or should not) so you may find your feeding changes a little and you have to add Cal/ Mag / Iron etc but honestly, I am on week 12 of a grow only using PH Neutral water and occasional tests and watching the plant to see what is missing or in abundance. The only issue I had was forgetting to add the right food for 3 days... And once you added everything, re-PH the water for your grow medium (6.5+- for Earth? sorry, I only grow in Coco, but I think that's about right). Just get a Growers PH Down, you wont need Up. Good luck and again, I really recommend buying an aquarium style R.O Filter (or even an inline filter), it WILL save you in the long run if your water is that bad.

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.