Brown Leafs?

Yellow damaged leaves like this will stay like that, they will not magically get back to normal looking leaves, no matter what you do. All you can do by giving treatments is to stop it happening to other healthy leaves in the future.

Your plant's nutrient needs are greater than what your soil mass is capable of supporting. I recommend using larger pots, the larger the better. Could also be a ph issue, but I would bet on the above reason.

first check if the brown spot is dry because i cant tell if its just dried out already or possible disease if yes no worries for now the leaves wont turn back to green any way so dont stress about that. you just need to check for new problems to see if the fertilizer worked its always hard to tell but i guess its one of 3 things: 1. your plant is far along in its lifecycle and this is the first sign of the end of flower (no need to feed) 2. ph related nutient problems (nutrients available but cant be absorbed/no need to feed) 3. regular deficency (no nutrients available/feed) if there is no new problems showing up great you did it for now if there is more and varying deficencys showing its probably ph and feeding isnt the problem so you should just feed ph regulated water its unusual to have trace nutrient deficencys if you applied your feeding regiment correctly most of the time its n-p-k thats causing trouble so dont overthink too much there is a site called growweedeasy.com with a great overview of disease and deficencys check that out and try for yourself to see whats most likely in your case as only you know what you did exactly for my part id just take it off as there is not much green left and it becomes next to useless at this point as here is no photosysthesis possible without the chlorophyll keep observing and good luck