First grow and problem keeps getting worse

The strain is jack herrer and it’s three weeks into flowering. The fan leaves have been getting brown spots then turning yellow and then turning a crispy brown. It started with the oldest biggest fan leaves and then started spreading. I watered the plant yesterday with calimag and a gallon of water that was ph’d to 6.5. I am growing in fox farm ocean forest soil in a five gallon fabric pot. Can anyone tell me what’s going on and how to stop it? I think it’s either lock out or maybe nute burn?

I'm sorry but I have no clue how to deter nutrient burn from Foxfarm soil

How do you address Mg in your grow? Is the CalMag you gave her, her first dose?

I'm guessing you mean 3 weeks from 12/12? that would be a common place for something like that to happen in your soil.the FF soil sometimes comes acidic and when you start to flower you run into problems. check the ph of your soil not your run off.and start there before you do anything else otherwise your treatment whatever you do would only be temporary at best.your bad leaves are not going to get better.let everyone know your results of the ph test and then someone can suggest treatment hopefully.who knows she may just start to grow out of it on its own.A little lime in the FF soil at start is always a good insurance policy for the ph problem and cal-mag later. best of luck to you.watch the new growth not the old dying stuff.