Cardinal Tetra needs Help (unsure of what to do)

[StripySnailGirl04]

Hello,

Yesterday, I found my last cardinal tetra stuck behind the filter with half its tail gone and an injury on the side. However, it was still alive. I let it out into the tank and turned off the lights to prevent the other fish from attacking it (I normally do this if they become aggressive and it works) but it didn't work. Now the tetra has no tail and half it's top fin is missing because I found it being eaten by the other fish. I separated it into a separate bowl floating on top of the tank water thinking it would die soon.
However, it survived last night and is still alive. I think it tried to eat a bit. I changed the water in the bowl and added a bit of food and java moss. It is still breathing quickly but it can swim reasonable well for a fish that does not have a tail. Because it survived the night, I think it has a good chance of survival for now. The lights are still off (I only turned them on to take some pictures below). It has control over where it wants to swim and seems to be regaining strength so euthanising it is something I don't want to do yet.
Due to some injuries, should I give the tetra some medicine to prevent infection? Maybe something against fin rot because the whole tail is gone? Will it be able to regrow its tail?

 

[GaryE]

That looks like a bacterial infection. It is communicable, and, sadly, fatal.

 

[StripySnailGirl04]

That looks like a bacterial infection. It is communicable, and, sadly, fatal.

Is it possible to cure it in any way? Any medicine you would recommend?

I just realised that it has white dots on its fins

 

[GaryE]

So eyeballing it from far away (not efficient, but), it looks like flavibacter, or 'columnaris'. It hits new arrivals, but can be dormant for months.

New fish, right?

I can only bear bad news. Maybe someone who lives in a place where you can get antibiotics without a prescription can suggest something, but it sounds like new fish with columnaris and possible ich. We all fight politely about ich treatments. I use malachite green ones as if you get them in fast, they clear ich. But they do nothing for the deadlier flavibacter...

 

[StripySnailGirl04]

So eyeballing it from far away (not efficient, but), it looks like flavibacter, or 'columnaris'. It hits new arrivals, but can be dormant for months.

New fish, right?

I can only bear bad news. Maybe someone who lives in a place where you can get antibiotics without a prescription can suggest something, but it sounds like new fish with columnaris and possible ich. We all fight politely about ich treatments. I use malachite green ones as if you get them in fast, they clear ich. But they do nothing for the deadlier flavibacter...

This isn't a new fish and I haven't had new fish for at least 2 years so it must've been something in the tank for a while that only infected the tetra now because it has new wounds?