Yellowing leaves with brown spots and curled down margins

Hey Good People,

I'm sure you've heard variations of this a million times before and I've trawled the interwebs diligently to find out what's going on but can't figure it out. I need your expert opinion! Photos are attached but here is the issue in a nutshell. My leaves, many from the bottom but also the top, are starting to yellow and develop brown spots along the central and branching veins. The leaves are also curling down along the margins and the tip. I noticed the curling before the brown spots and yellowing became apparent, and I'm not sure if the issues are related. My entire plant isn't blighted yellow but the problem is spreading.

Photos:

This is an indoor plant, grown in pure Pro-Mix, watered every 2-4 days or whenever the top inch or so feels dry to the touch. I've switched over to a flowering light cycle about two weeks ago.

I have not added any additional nutrients since a few days ago when I added some Ca-Mg and Fox Farm Organic Big Bloom concentrates along with the water, thinking it's some kind of nutrient deficiency. I've always been watering with tap, not pH controlled, and before I started with the nutrients, I tested the pH of the soil which came out to be 7.3 or so. Since then, every time I've watered, I have added some lemon juice to lower the pH to 6. It's currently sitting at about 6.8. The Pro-Mix I used advertised that it will feed up to three months, which is why I avoided supplementing before this week. I have some pH down solution on the way, which I will substitute for the lemon juice upon arrival.

Any input is greatly appreciated. This is my third grow and the last two plants suffered the same issue and flowers were shit. I thought it was because I heat stressed them too much but I've kept the temperature steady this time at 80-85 during veg and lower now. Humidity is being kept at 45-50.

Isnt pro mix entirely devoid of any nutrients? Looks like mid stage N deficiency. Unless your water has a good mineral make up your going to need to continue the cal mag and N indefinately. These are my opinions only and are not based on any factual information . Also this opinion may not be the opinion of others in the community 80-85 is too hot in my opinion.

The Pro-Mix I purchased had some food.

I shall decrease the temp next time but do you think the initially high temperature had any effect to cause the changes in the leaves? Plants from my first grow suffered 85-90, which is crazy.

I will keep on with the nutrients and hope for the best. Thanks for you input!

Your new growth appears fine, looks like you've managed to fix the problem which most likely was an N deficiency. Lemon juice is good for lowering pH, you can also use vinegar, as it does the job as well.

Temps should stay under 85F, so you should be good on that note.

Keep up the good work and welcome to RIU!

Much appreciated and thanks!