What are your exact readings? Have you tested your tap water for nitrates? Are you sure its fin rot? Are all of the fish affected or just a certain one? Can you post a pic? How long has this been going on & what have you tried other than clean water? Sorry for all of the questions just trying to narrow it down a little more.
I'm wondering if you have an overly aggressive fish that may be bullying/nipping the others. Are your nitrates always at 40 or does it take a while for them to get there? You do weekly 50% +/- water changes? I would throw in another water change or two to bring down the nitrates to <20 if possible. Fin rot is usually kept at bay or will go away with normal weekly water changes. What is the full stocking of the tank? Sorry, it just seems you're pretty much doing everything I would normally suggest for fin rot so I'm trying to come up with some ideas.
Just recently started taking better care of it. I'm in law school and teach as well. Oh yeah and have a one year old. Tank gets neglected during the school year. Only aggressive fish is a syno cat, but he stays in his log during the day. I guess I just need to keep the water super clean and see what happens.