WHY MOTHER NATURE WHY?? help a newb?

Hey all,
silly newbie here wth newbie issues
3 plants outside during the day inside unheated garage at night for darkness.
Mix is 1/3 top soil, 1/3 organic Veggie 1/3 Miracle Grow..... yes, I know NOW.
Never added any nutes.
Tap water, ph runs 6.8 in the soil.
My super cheap ppm " gauge" only reads low med high..... mine reads very low.
Have been slowing down the watering because I have been told previously I overwater, but these last few (90 degrees)
Hot days have screwed that up. Today they are reading 0.5 on a zero to five moisture scale.
Does anyone have a diagnosis or any thoughts...... everything I can learn is greatly appreciated!!!

Check the PH of the soil. The PH of the tap water itself doesn't change the soil ph. The soil isn't 6.8.

Thanks for the reply, that was written confusingly.
But I have watered all along with tap water only.
And yes, in all my testing, in typically 3 different locations on the plant each night , the SOIL Ph has consistently read 6.8 any thoughts please?