fungus, bacteria, or just an injury

If it were only the tail, I wouldn't necessarily worry about it. Would wait and see. From the picture, I see some purple ares and spots on the body of the fish. This would be my concern. I think maybe you were too quick in trying to cure the fish if you don't really know what the problem is. I rather use less than more.

I am a big proponent of using salt to treat fish. Some fish don't take too well to salt, but works most of the time. Don't know what other fish you have in there. If you have a spare tank, move the fish out. If not, if your fish can take the temperature, I would raise it slowly to 84-85F and start treating with salt-sea salt or epson salt. I would would try going up to 1tbs per 10g. Dillute the salt in a container with tank water. Add slowly some to the tank. Wait few hours and add some more. Continue until you finish the mixture. Don't change water for a week and after that do 50% w/c.

BEFORE YOU DO ANYTHING. When you started using your med, did you have any charcoal in your filter - you should have remove it before. Now to get rid of the meds, use charcoal and w/c of 50% every day for a couple of days. If you decide to use salt, now is the time to start using it.

One more thing to consider before you do anything and for the future. How big is your tank and what fish you have in there and filtration and how often do you change your water and how much?

I would answer my last question first and go from there.