Koi fin rot? advice please

I've only just noticed this fish swimming oddly and hiding in amongst
the shallows.

On closer inspection the lower half of the tailfin is missing I
searched around and think it may be Fin rot.

http://www.landshapes.btinternet.co....804-175907.jpg

Do you think it is worth treating or would euthanasia be the kindest
solution.

If it is treatable what would you recommend?


the fish was bought along with two others about 12weeks ago and is
3-4inches long the others seem fine.

thanks


Bob