Fin Rot Or Not?

[LacertaIlla]

One of the guppies I bought the other day has something strange going on with her tail. 
 
Yesterday she was fine. Today she looked like this.
 

 

 
It is not from the other guppies biting her tail (it was happening to one of my other females, its stopped now and her tail is healing up nicely), its hard to see in the photos but the end of her tail is still there, its just gone extremely see through, and it's not like when a fish's tail is healing either (the 3 fish that I've had that've had tears in their tail have always been kind of murky/not entirely translucent while healing).
 

 
The part where her caudal fin attaches to her body has also gone a strange grey colour.
 
Is this fin rot? Is it something else? How do I treat it?
 
Thanks.

 

[LacertaIlla]

Update; I've just pulled two more dead fish out of the tank. That's a total of 7 fish I've lost to this thing. Some advice on how to treat it would be helpful.

 

[LacertaIlla]

I've just gone and bought some Melafix, some Tetracycline Hydrochloride and a "multi cure" formula containing Malachite Green, Methylene Blue and Acriflavine. I've moved the fish into a 20 litre tank (5 gallons) and I've dosed the fish with the recommended dose for fry (I have 5 fry in a breeder box in the tank as well), I've also dosed my 140 litre tank (39 gallons) with melafix, as I've got 3 mystery snails still in there and I don't know if the other chems will be harmful to them.

 

[LacertaIlla]

5 more dead fish this morning. I'm left with 3 adult guppies, 4 molly fry and 1 guppy fry. Surprisingly, the fry don't seem to be affected by whatever it is.

 

[Baccus]

I have used a Science Product Multi-cure (I forget exactly which one I know the company makes two types and I only ever use the one) before on my shrimp and loaches without causing as far as I could see any issues or deaths.
 
When you put the fish being treated into a 20 L tank did you use their existing tank water? Or did you use all new tap water? Is there a heater and filter on this 20 L tank? without a cycled filter the fish will quickly foul the water and you will then be battling ammonia poisoning as well as the tail rot/ fin rot issue.

 

[LacertaIlla]

I used new tap water, de-chlorinating it obviously. There is a sponge filter and a heater in the tank yes, the temp is set for 20°c as I've read columnaris doesn't like cooler temperature. That's what I think I'm dealing with at least. I've only got a single remaining adult, Merle, and the fry left. I'm in the process of doing a 50% water change as I type this, and I'm going to re-dose the chemicals once I'm done.
 
The multi cure I have is by Blue Planet, they were the only brand of meds on the shelves, minus melafix and pimafix.