Heat? PH? Something else?

This plant is in soil under 84W of cfl, temps max at about 80F. Leaves are kinda stiff and papery, and turning black.

Using fox farm trio... water from a brita.

So what do you think?

How often do you water? Drain holes in pot?? How close are cfl's to leaf??? Humidity???? PH of soil????? I'm going to go out on a limb and guess you haven't used nutrients yet?????? How big is pot, maybe root bound and moldy??????? I'm leaning towards mold, like if your plant is rootbound and too humid in room/box/etc... You got a fan on the plants???????? Gotta answer the questions above for us to better help you bro, peace and good luck!!!!!

Wouldn't mind seeing the plant from afar & the main growing tip... would help. The color changes you see are actually a very deep rust color. The dieing tips curling up & the upward cupping of some of your leaves are all clear indicators. Dissolved epsom salts to immediately correct. Likely you have other issues as well. My primary concern would be pH of your feed, medium & at run-off. ditto on above... the more details you can offer the better able to address your root problem. Mg deficiency is clearly the immediate issue you are concerned with.

Ok, I flushed with 6.5 ph water and saw that the runoff ph was low, around 4 or 5. Flushed a few more times with water around 7.5, and along with a light dose of all fox farm trio, also with a high ph. After using this, runoff was still low, so I flushed again. Its still less than 6.5 but I think I'll leave it for now so I dont rinse out the ferts that just went in. I have 2 120mm pc fans running 24/7, 1 in, 1 out. The plant is about 18" tall in 6" pots. I've been using nutes all along, but probably too much. I am re-using old maricle grow, is the fox farm nutes enough to make up for this? Thank you so much, let me know if you need more details. +rep if you help me fix it.

Re-using old miracle grow soil is probably not a great idea. It will contain old roots and nutes and could harm your plant. I would buy some Foxfarm original or at least some new potting soil and transplant it. The old soil contains sodium from old nutes and other contaminants that could harm the plant. So, I would just repot it. Good luck holmes.

Oh, and six inch pots for a plant that size are probably too small, I would re-pot in at least a five gallon pot. I have a plant that is 24inches in a ten-gallon pot and the roots fill the whole pot. Peace.