From Your Gums And Teeth

 Fin rot! 

This case is likely to be bacterial and not fungus as the rot is not uniform and has a ragged edge.

Both pseudomonas frequencies \(174\,Hz\) and \(633\,Hz\) did not seem to work.

A blast of dental bacterial frequencies,

Dental Bacteria 95.75, 175.00, 269.71, 355.08, 842.00, 937.41 Hz

that, I use for myopia, cause the fish to hang tail up.

On removing the Dental Bacteria frequencies, the fish swims normally,

this particular rot is caused by dental bacteria and when a fish hangs tail up for a long time use this set of frequencies too.  It is likely that the toxins from the dying bacteria causes the odd hanging-tail-up behavior.  Under normal circumstances, the disease has to progress much further to cause such a behavior.  With the resonance frequencies, rapid bacteria death from rupture, elevates the level of toxins as if there are a lot more bacteria. 

The fish may swim normally even with resonance frequencies playback, later.  

But the rot persist...