Looks like PH fluctuation - you stated your possible water ph and you are uncertain of ppm . Most municipal ( city water already has calcium / mag ) , and so does most nute lines. You also stated you wanted something to “ check “ it.
First , what organic potting mix are you are using?
It may or may not have ph buffering amendments.
If not, then it is up to you to narrow the ph of water going in ( ph / ppm pen ) to work with medium ( 6.5 + or - ) . Symptoms indicate the PH issue. Plus you have uptake issues - purpling / plant color.
I noticed you stated 80° temp - led grow like higher 80’s ( 85 or so ) ( which helps uptake ) cooler temps slows uptake and growth too.
If it was my plant i would increase ( light cycle on ) temperatures - narrow the ph to meet needs and feed accordingly.