Hi, Jessica.M,
so sorry about your problems, and, boy, can I empathize.
I've recently encountered that black fin rot for the first time, and have been struggling with it for several months myself, although the fish about which I'm most concerned was actually infected through splashing from the tank beside his.
May I ask what you're treating yours with?
I've tried Maracyn and Maracyn 2 and several wide-range antibiotics as well as salt and Pimafix (the last in the beginning) and wanted to try Jungle Fungus Eliminator but seem to be unable to get any out here.
Have you tried that yet?
I don't know how long you're treating for or with what, but it seems evident, as the black rot recurs whenever you back off, that the treatment you're using is effective but needs to maintained longer.
And each time, the potential for resistance to meds develops further...
My Graffitti was sold as a half moon as well, and would have been if his top outside ray wasn't bent the wrong direction - they're evidently far more susceptible than most as black fins came in on some tetras, Harlequins, a female betta and a crowntail betta I've bought over the past few months from the same source, with only the Betta fins being eaten away at all at the time of purchase and improvement on the bettas since, if no cure yet on any fish with the exception (knock wood) of the crowntail male, while on Graffitti the progress appears inexorable, eating away half his tail from one little spot appearing high up and just now having reached his body.
I have hopes for the Maroxy, though, which I just managed to get and switched to last night, and I don't think it's wishful thinking that some overnight improvement appears to have occurred.
If any miracles happen over here, I'll let you know, in the event you haven't tried this yet and can get it in your area.
If it IS necessary to put down your fish - freezing is PAINFUL as well as frightening.
Clove Oil is used by some as a fish anesthetic, and a few drops mixed in water and added into an ice cream bucket or other such container of tank water will cause the fish, once put in, to gradually slow and go to sleep - too much evidently burns or stings as they'll struggle.
But it won't actually kill many fish at that dosage, although bettas can drown while unconscious and unable to surface breathe, and the only way to be sure is to add LOTS once CERTAIN that the fish is out cold and hasn't moved for some time.
This is the only way I know to reduce terror and suffering to the lowest possible extent.
I leave the body (in a quiet corner where it's easy to check) until it floats, so as to avoid any risk of an unpleasant, and slowly deadly, awakening.
I also keep bodies (of all fish, in disposable plastic cups) until VERY obviously dead, having been warned that bettas, especially, can appear totally dead only to revive once flushed or thrown away.
But as long as there's hope and the fish is not suffering severely, one can always try, as miracles do sometimes happen and a last-minute save can occur.
And if you're using something that is effective, even temporarily, please let us all know, and at least there will be something that others can try as well, even if it's too late for some.
This black fin rot seems to be something new.