This is a good one to let your imagination really work for you. It is called gut reaction.
I've used activated Miracle-Mineral-Supplement in a nasal spray bottle with distilled water. It did not sting; but it also did not help me. My nasal fungus problem must have been internal, within the sinus tissue. I admit that Neti pots and WaterPic devices have been worthless to me as well.
So, it would not be harsh at all. It would be highly diluted in a Neti pot anyway.