Brown/rustic spots

Guys it's my firt grow and everything's was going great till these spots started to show. They seem to be appearing on the older and lower leaves first. They start in the middle of the leaf and move outward. Only one leaf shows brown on the tip.

-hempy type with perlite/verm
-temp is constant right under 80F and have a fan blowing inbetween cfls and plant
-had recently been watering with 25% nutes and did 1 watering with 50% nutes. Than stopped. Also was attempting to buffer ph with some jungle fish tabs but I stopped that as well. Oh and I'm using fox farm.
- they are About 3 weeks old.

Lemme know if you need any more info.
Any help is appreciated.

Here's what it looks like as of today.

After some research I've found some symptoms that match for potassium deficiency or Ca/ Mg defieciency? But the picks showing examples don't quite match mine. What do you guys think?

likely to be calmag. what water do you use? if RO, give some calmag.

Also I don't think you should be giving just water in that medium. at least just a bit of nutes is needed.

Yes i do use R/O. I bought some lime from the local lowes and that had some Cal/Mag i mixed in about 1tsp of it. That was about 12 hrs before i took the pic i provided. Maybe it just needs more time to heal? OR do i need to add more?

I water one day with 25% nutes and the other with plain water to help wash away any salts, repeat. I was working my way up to full strength, but this happened and not wanting to cause any further damage i reverted back to what i knew was safe which is 25%.

yes that is good method
so yeah maybe you just need more lime. or just use that product.