Remove the gill cover with a pair of scissors. Have a look at the gills. What colour are they, {pink, red} any white patches, any watery appearance.
Skin & Gill flukes are O.3-O.5mm in size. They differ in look. You can even do a scale scrap & have a look there too if you have a microscope. Try & get hold of a good text book that has disease pictures, therefore easy to compare & see if the gills are healthy, riddled with flukes etc.
I think you left the flukes to long on your fish. As you have lost others, I would do a water change on your tank & keep treating with prazi.
I looked up Furan-2 too,
Your fish have flukes. That isn't bacterial. Treat with prazi, or with what Alan stated.
If you explained what you said your fish were doing to the lfs....why did they sell you melafix, then furan? Oh well.
Frenchy