Serious Kush Micro Grow

To ME, first of all it's not an excess of anything but rather a deficiency... and I'd be willing to bet it IS potassium ... which leads me to think your PH is not in a good range so that's the first thing I would check... Potassium is absorbed best in soil at a pH level of 6.0-6.5 (it's different for soiless). I would shoot for a PH of 6.2 so you don't run into issues with any other nutrients. Do you check the PH of your runoff? If you don't, you need to do so as you might have some lockout issues and need to do a flush if the PH of the runoff is badly out of whack (and then get back to your normal water/feed schedule) ... If you don't have a PH pen, the best advice I can give to ANY grower is to invest in one - having the ability to balance the PH accurately wards off many, MANY grow issues and resolves issues that DO crop up. Good luck!

your plants look N heavy so dial the N down a bit at this stage and you can hit it with epsom salts a couple of times but its not gonna bring that leaf back most probably. once the plant starts cannibalizing leaves it not a reversable deal though a sensible dose of epsom will stop its spread.

Thats not a toxic. Its a def. Most like Magnesium a later stage def. Anything with trace elements in it. Bring your ph around the 6 mark. Up your Mag. You can add epsom salt which will have mag sulphate or your need a calmag. Good luck.