Spotted leaves on auto fem, need help

[ArcticStrain]

This is my first grow and I have two plants of two different times. Everything between was done identical and the one started showing spots early and now starting to see it on more leaves. The second plant is perfect with no issues. So far, ph of water is around 7.5 so not ideal, started now using lemon juice to drop ph but found nothing else at this point. No bugs or mites, no fungus or anything. Light is 24 inches away and fan blowing. Consistent 50 to 55 humidity and 70 to 75 degrees.
Any advice would be great.

https://imgur.com/a/UhaIbzw

[Gingerbeard]

What you see looks like old pH damage. I say old because the damage is brown and dead in those areas. From the pictures, it is only on the lower leaves, which is not generally an issue. Old leaves often die off.
7.5 may have been fine for veg, depending on your grow medium, nutrients, and water quality. They and your medium are getting older, so you need to get a handle on in before you get much further along.
But you need to seriously consider a stabile pH adjuster.
What are you using for medium and nutrients?

[ArcticStrain]

Went with Pennington's rejuvenate organic soil. Preferred it over chemicals and came loaded with worm casings. Haven't added much nutrient at this point. I read elsewhere it is perfect for auto flower as the soil load has enough nutrients in the right quantity.
I will pick up some ph adjuster. I went with lemon juice to keep it natural for acidity.

[Gingerbeard]

Premixed soils can be hot to begin with. That means they are loaded with nutrients and junk. They provide everything a vegetating plant needs, including pH adjusters. Over time, watering flushes most of those first hot nutrients and your soil cools. By the time flowering comes, most premixed soils need flowering nutrients. Running autos in premixed can be better than fem and photo just because the nutrients have not had as much time to flush.
I do not know Pennington's. What I am saying is generally how it goes with premixed.

[Rwise]

For me lemon juice did not maintain the PH levels, they drifted back to what it was before using it, same with vinegar. It depends on what buffers are in your water, try this PH a batch of water, let it set overnight and test it again. Myself I use sulfur as a PH down buffer and dolomite as a PH up buffer, and when I PH water I use sulfuric acid (must handle with care it can burn the hell out of you). (Sulfur + water + time = sulfuric acid)
Good luck!

[Bluey]

Isn't 7.5 a bit high?

I was an outdoors soil grower and I used straight rainwater and didn't have to adjust the pH as it was around 6.7 give or take and the soil would buffer it when/if required.

Lemon juice (ascorbic acid) holds steady for me in my indoor grows with my water, nutes & supplements/additives but for some other mixes it can be disastrous and result in a reaction causing quickly rising pH and a rotten smell too, like something died in the nute tank and is fast decomposing (likely hydrogen sulphide). I have run into trouble with it in the past with an incompatible cal/mag suppliment. If your just adding it to water you'd think it should be fine but it also needs to be reasonably compatible with your medium, which it may not be. Allowing a small amount of runoff and testing that against your input pH will tell you that easily enough if it is rising in the medium.

[Rwise]

Yes 7.5 is out of range, in flower look for 6.5-6.8 in soil.

[ArcticStrain]

Adjusting the pH seems to be working. I gave it 6.5 with the Lemon juice and leaves are turning back to the green with the spots shrinking.