I could not view your picture -link broken.
But you may be able to save your fish by removing it to a "hospital tank".

This year I had one of several small fish I had added die in the pond
only I did not know it. Its decay cause one of my established fish to
get fin rot. I removed it to a separate hospital tank (a large garbage
bin).
I filled it with pond water, added salt .33% (2.5lb/100gal) and raise
the temperature to increase metabolism and added an air stone. I also
treated the water with potassium permanganate to oxidize any bacteria.
I also treated the pond as well. In two weeks I was able to return the
fish to the pond and he is doing fine.

Good luck with yours.

wrote:

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