What's a PH pen? hahaha been growing for 20yrs now and have never used a PH pen or nothing..
Tap water, filled and allowed to sit out uncapped so it allows chlorine and other badness to evaporate out... I've lived in 2 different locations now and because neither uses well water it's safe for me to do something like this.. some waters you can't.
To get your tap water tested, simply take it to a pool supply store. they will test if for free and tell you what it needs to be brought to ph neutral. Most can be repaired by simply adding a little lime, but I never do that either. I feed with a light hand, buy good soil and for the most part have no issues. can't be just luck as many many others use this same method to water..
not suggesting you do, just saying don't over think it. Hydro of course you absolutely have to do this, dirt though is
much more forgiving.. luck man