On Sun, 26 Feb 2012 12:37:32 +0000, Spider wrote:

I'm wondering if your unidentified and uninvited visitor could be a
dragonfly larvae. At a stretch of the imagination they might be likened
to a fish, colouring sounds appropriate, and they are voracious feeders.

From memory a dragonfly nymph has visible legs on hatching as do other
invertebrates such as the great diving beetle larva (these are really
voracious, even tackling small fish).
--
rbel