My opinion..... not biological.
It looks like chemical burns to me. I think you have some sort of chemical in your tank water. Something was sprayed in the house, near the house or something and its now in your water. (Herbicides, fungicides, insecticides... in the yard, house or neighboring farm... a flea bomb dropped in the house, something was just painted and now there are paint fumes.....) The list of options is huge....
Again, this is my opinion.... I would do MASSIVE water changes.... start doing daily 70%, or even 2 or 3 x daily. Keep up the salt with each water change. And most importantly get BRAND new activated carbon into every orifice of your filters. Change the carbon out daily.