Well, others may will have different ideas, my experience is that even with the best fertilization you can have this on your ferns. Maybe this is a fern disease? Could be.
I am not saying it because we sell ADA products, but my personal experience is that ADA Phyton Git helps with this kind of problems.
It's not when you have spore on your ferns or holes because of the bad fertilization. This is just when the leaves get black here and there or on the edges. For me the products above helped. You can get that from TGM i guess.
But looking forward what others say about it.