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.