Let's focus on the single issue that has been proven with certainty: elevated nitrates. The fact that reported water changes don't lower them, even though your well water has zero nitrates, hints at the possibility that your filter or other decor are the source. Either that, or overfeeding. Matter of fact, I don't think those fish need to be fed twice daily. My suggestion would be to change water daily and starve the fish for a few days. They'll be fine. See if that does anything.
Good luck, keep us posted.
Good luck, keep us posted.
