Rust-colored margins and dots. I would like to go back to fertilizing but I am afraid that it is an excess of potassium

need more infor to be certain. looks minimal at moment which means you can be patient dealing with it. tip burn can have more than one cause... if you have a more resolved understanding of what you feed, the answer will be apparent in simpler contexts liek this one. (as opposed to a bunch of things coincidingn or more significant damage at faste rate etc)... Too much P / Too little K / too much S / too little S / even high Cl which is not likely. Lockout from other nutes being too high in concentration and pH related reasons are possible too, but in those cases you often see more than one thing going wrong and usually not so subtle symptoms, ime. Check out manic botanix nutrient ppm calculator (google it). use guaranteed analysis labels to tabulate what you feed of n/p/k/ca/mg/s ... compare to my breakdown in any given week... ballpark conepts only don't freak out if it's a bit different... BUT, someting may stand out drastically and that is a good place to investigage. soilless should be fed with every irrigation. get 10% runoff religiously and you will never have mineral buildup in substrate. know what you put in and you will very easily react and adjust when you see anything... and as you do so you make a more perfect solution each time... fewer issues in future if it is well-balanced ratio of nutes at proper ppm. 650-750ppm is a good range for a regular feed.