Is this leaf septoria?

Do you have mold/mildew or some kind of fungus you need to deal with? A good organic fungicide and pesticide you can use is neem oil (according to recommended amounts) mixed with baking soda (1 teaspoon/liter) in tomato leaf water (or water with tomato leaf extract) the neem oil is a good fungicide and pesticide, the baking soda is very effective against fungi, and the tomato leaf water are excellent against leaf munching pests. As for the plant in the picture if you're saying that the diet was different with more potassium than what is mentioned then try checking the pH level of the runoff as well as the irrigation to make sure the soil is in good condition not just the water. If the pH is fine then the next option is Calcium Toxicity which inhibits Potassium uptake - from the looks of your plant it is very unlikely but still possible, to deal with that try lowering your "Micro" dosage by half. Hope this helps! Good luck and happy growing :)