PH Up, PH Down questions

I'm in the nutrient forum because I figured you guys would be the right guys to ask. My tap water comes out at 8.1 so when I feed my ladies after mixing my nutrient solution I have to add some PH down to get the solution to 6.5.

Now here's the question. When I'm giving my ladies plain water should I still be adding PH down to get my PH to 6.5? Or is it harmful to use water+ph down? Someone told me you can only use PH down with nutrients.

Also I'm using pro-mix soilless mix. I was under the understanding I should be setting my PH between 6.5 to 7.0. Can you guys confirm this.
 
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