I know that you said that you have dealt with fin rot before but did you know that there are two types of fin rot?
Bacterial fin rot- This type of fin rot seems to the the most common type, the fins break away over a period of time, that is the fin damage is gradual often occurring over a week or two.
Fungal fin rot- This form of fin rot is extremely aggressive, the fins are eaten almost entirely away within a day to a few days.
Otherwise is we really are not dealing with fin rot the only other thing that I can suggest is to check ammonia levels in your tank because ammonia poisoning causes the rapid desintegration of the fins.
Maybe test the ammonia levels and then if readings are at 0ppm then maybe post some more pictures so we can get a better side-on view and go from there. Goodluck