Hey there, that's a magnesium deficiency, you can tell from the yellowing between the veins and the burning / discoloration on the sides of the leafs affecting the older leafs first. It has nothing to do with NPK nutrients or light burn, but it's actually due to the high heat and "intense" light schedule, see : magnesium is used to regulate metabolism (which is itself regulated by temperature and humidity through something called VPD) and photosynthesis so with high heat and lots of light you'll get magnesium deficiencies as the plant isnt able to absorb enough quick enough. So you've already started adding some to the feed, that's good, maintain that. I would recommend also making it into a foliar spray like 3ml/L and just spray it on two to three times a week shortly before lights on or shortly before lights off. uptake by the leafs is 85% efficient compared to 15% through the roots. You should really try to get those temps down (have you considered an evaporative cooler?) and maybe even reduce the light schedule a touch too reduce the stress on the plant. Hope this helps ! 🚀