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The rapid breathing is a serious symptom. It could be velvet or Brooklynella as mentioned. The eroded dorsal fin is also a problem, that is most likely a severe bacterial infection, but there is also a rare fungus that can cause that.
The treatment of choice, is formalin, but I doubt you will be able to find any of that available in time to help your fish (various governments around the world have restricted its use because it is pretty dangerous to people).
Less effective, but a lot easier, would be to try a five minute freshwater dip. That may help temporarily with the rapid breathing, and may help with the fungus. It will not help if the issue is bacterial.
Jay