What ph should my run off be at?

[Butters]

I haven’t watered to run off yet but one of my plants has started to lighten around the edges and I don’t know why .

Yellowing on the edges

I’m growing OG Kush autos in potting mix under led’s. This one is about 3 weeks old and last week I noticed a slight yellowing along the edges of the leaves. Under the led it looks much worse and yellow but in normal light it’s not so bad. The soil ph is about 6.9-7.0 and I’ve just been using demineralised water at 6.0 ph. I’ve used a seaweed foiliare spray once a week. Any help would appreciated. The original 2 cotyledons seem to be shrivelling up. I have some chicken manure, flora grow trio a…