Treat using Myxazin.... put the fish into a smaller tank.... increase the heat to 82 - 84 C..... No filtration just clean water from the main tank to start..... Add dose ever day for 7 days and then every 2 days if you still need to treat. IMPORTANT... Do not feed the fish as theres no filter for the first 3 days... as soon as its eaten syphon off 10% making sure no uneaten food is left in the tank. Keep treating and feed again on 6 days.... syphon off uneaten food immediately... Keep going till the infection disappears.
I would do a 50% water change and substrate vacuming then treat with API Fungas Cure. That looks to be too severe for Myxazin which is basicly Melafix IMO.