Goldfish is looking pale, and tail is worn...

Hi,
I have 2 small goldfish in a 15 litre tank. One fish is looking unwell
this morning. I believe he is a Comet as he has always had beautiful
long fins and tail. Today his tail is a lot shorter and a little bit
jagged. However, his other fins are fine. He normally has all his fins
fully out all the time, but today they aren't as much, his dorsal fin
is down a lot - possibly indiciating he isn't happy (?).

He has never been a strong gold colour, but I think he may be looking a
bit paler.

Is this fin rot? Or something else?

And suggested treatment if possible please!


I have moved the other fish, who is short tailed but looking very
healthy, to their old tank. Note they have changed tank, that was about
a month ago. They seemed happy and excited with the change and this is
the first sign of any problem.


Thank you in advance for any help

Deb