Persistent leaf problem

I'm currently growing Shit (Mr. Nice), and I'm having this problem with my babies.
Growing in soil, watering with plain tap water (original ph: 8.1) that I ph down to 6.5 (no additional nutes)
I didn't have a ph meter for the first two weeks so I had to feed them with 8.1ph water :/
All of my plants have the same yellowing on the older leaves and on the tips and I don't know how to treat it!
Temps: Around 85-86 Humidity: Around 40% Unknown soil.

Ph of runoff water is around 7.46

Here's a better shot.

how often do you water?

Every 5-7 days. Plants are in 1 gal. pots

They are healthy so water more often, this could be caused from letting the pot dry too much too often.

I hope you're right!