Stiff thin tail?

[Stella]

I have seen this a couple of times, what is happening when a fish's tail goes stiff, strangely thin and awkward to move? I am talking about the whole area or 'body' (not just fin) from behind the dorsal/anal fins back. Not plain old clamped fins.

I had one do it quite suddenly a little while back. A friend who has been into fish for a zillion years and should know what he is talking about said it was a gonner but it came right. Now I am suspicious about another fish....

Thanks for any ideas.

[midge]

Ive had a couple of platties and swordtails do the same thing. they go all skinny and it looks like an effort to swim, and have the funny fins etc. i asked the LFS and she suggested putting some rock salt in the aquarium. the latest one perked up a bit, but still is skinny. sorry not much help but maybe an idea.

[Caryl]

Assuming the fish is still eating well, if it was skinny I would assume some sort of internal bacteria or parasite.