Even CO2, like as in none being added at all can do this.

This is a chemical type of burn.
It's not a dirty filter, organic build up etc.
Even Chlorine burns are not quite this bad.

Was the KNO3 really KNO3 and not Triclopyr(A herbicide)?
Stump removers are sometimes sold with that or another herbibide.

Tap water differences are the other thing......sicne it was correlated with the water changes.

So tap or the mix up of stump remover.
Maybe the spouse.

Something that beats up the swords inside of 2 days that where doing well is something like adding lots of salt etc type of thing.

It's not nutrients/CO2/light................

Tap can change when they switch over, but the dechlor tends to get any of that unless they switched from Chlorine to Chloramines and he uses Sodium thiosulfate etc rather than Prime/Amquel etc.

Regards,
Tom Barr