Calcium deficiency or overfed?

Were all saying basically the same thing in our comments. I agree with your initial thought, looks like heat or light stress but that depends on how long this has been going on. To have that much yellow on the tips and areas dying, if it was light or heat stress , all down the edges of all those leaves they would be curling up and almost folding in together. I do see some yellow tips down inside the plant as well as yellow between the veins. First and foremost with trying to diagnose is making sure your following the basic requirements as I state later in my comment. So it appears the lower leaves are doing it to then I'd suspect lack of Phosphorous. That also fits with the leaves appearing dark. The dead area along the edges could be it's because lack of Potassium but potassium would also stunt her growth. This is also signs of Magnesium deficiency. If the basics are fine, how you water, light distance and power, PH, nutrients and amount being feed, air circulation, temps and humidity, then I'd say she's lacking the proper nutrients. If you have been doing all this then I suspect lockout in the soil. Test your run off TDS or just do a flush.

Personally, I think the PPM is a bit too high. 1200–1400 seems like a lot, especially if the plants are already stressed. I’d aim for around 900. But for now, I’d just stick to pH-adjusted water at 6.2 and see how they respond. But it might already be too late.

https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg looks like a few things, maybe Ca and also maybe K issues could be some Mg, over all multiysmptomatic if caused by lockout or ph issues is the question.

Hi sieht schon nach Kalzium Mangel aus, wenn du kein CalMag gibst mach es, ich gebe es immer auch wenn ich kein Anzeichen von Mangel habe, einfach als Prävention. Kalzium Mangel beginnt am oberen ende der Pflanze das heißt die neuen Blätter bekommen es als erstes. Was ich immer empfehlen kann ist eine Kontrolle des runoff um auch andere Probleme auszuschließen wie zum Beispiel eine Nährstoff Blockade. Dein Runoff sollte ein PH wert zwischen 5.8 und 6.5 liegen und nicht extrem von diesem abweichen, er sollte auch ein EC wert unter 2 haben damit bist du sicher das alles im grünen Bereich ist und es wirklich ein Mangel ist und nicht eine Überdüngung.