The brown spots makes me think it’s a potassium deficiency. What medium/soil are you in, you said your runoff was a little over 6 so it sounds like it’s in range for soil.
Sunshine#4 soil, tap water, calmag ever couple weeks, feeding 20 20 20 miracle grow in the veg at 1/4 recommended amount, lights are 12 12 now for a week I have fed 15 30 15 miracle fro ultra bloom at 1/4 strength, the problem seems to start after a couple weeks under the big lights starts at the bottom on big leaves and now into the top fan leaves
Pot bound IMO. Re pot into something larger. Deficiencies are gonna show when the root mass is tight. Plus; that one leaf is just a beat up old fan.
It sounds like you are being cautious with the Miracle Gro. That’s good as it’s not PH buffered for weed which makes it harder to use. There are also micronutrients missing that cannabis likes. If you have a TDS meter I would definitely use it when mixing nutes.
I don’t think repotting at this stage is an option but I think I am going to change the food
Pretty sure its a cal mag deficiency, i was dealing with the same issue a few days ago. just 2 tablespoons of calmag per gal and should help her recover quickly !
I think I will add a bit of some bloom booster I have it has a lot of phosphorus in it I’ll go light and hope for the best
I use cal mag twice a week. With the hiigher thc strains the hungrier they get
I only water every 4 to 5 days