"Graham Ramsay" wrote in message ...

We had a guess at Chilodonella but as you say,
definite diagnosis isn't possible without looking
at the bug through a microscope.

Agreed. Still, checking on the net, Chilodonella is a better match for
the observed symptoms than my guess, Costia: the size and shape of the
parasite is right (0.1 mm & flattened), the slime production/sloughing
being heaviest on the 'neck' of the fish, and that it's easily killed
by formalin.

Another possibility along the same lines would have been Trichodina,
but it's said to be resistant to formalin, and to cause ulcerations.

I think I'll provisionally decide that this was a case of
Chilodonella.