finrot?

[markb]

I'm pretty new to this and inherited 5 fish from some friends who moved to tokyo. no idea what types they are really.. mostly pretty standard looking goldfish but with really really big tails, one of the ones with massive bug eyes and a white slender looking one also with a massive tail.

the big white one started losing bits of its tail a while ago and generally moping around n stuff, so i transferred it to a travel tank temporarily and gave it some finrot treatment. after a day or two it seemed to perk up so went back in with the rest. after a day or so it'd be moping around again and swimming/floating sideways so back in the travel tank and so on. unfortunately that one's now dead.

couple of days on and the biggest of the rest of them has started with the same symptoms.
only the tail seems to be disappearing, none of the other fins. also seems to be swimming more sideways that usual. all other fish seem fine.
i'm tempted to do the same again but really dont' want it going any further.
i have an undergravel filter and do a 30% or so water change every couple of weeks like the previous owner said but not too sure what to do.

can anyone help?

[markb]

is the closest pic i can find to it on the internet if it's any help.

[markb]

thanks for the reply and the welcome!

tank as about 2ft x 1 x 1

had a look on those links, and it sounds like fin rot to me.
think i need to test the water. as of this morning he also has some white dots on the tail where it's been eaten away.
i'm also wondering if a couple of the smaller ones have been nipping at his tail and that'sa what started it off.. hmmm.