parasite ?

Having recently found a suspected parasite on one of my goldfish in an
aquarium, and treated with methalyn blue, the parasite? came off and left a
scar on the fish we have caught the parasite it was swimming about in the
water quite freely. It is a green mottled colour about 5mm in diameter and
0.5mm thick and appears to have many legs and swims with a pulsating
movement flipping over occasionally.. Has anyone any idea what this might be
please , I have searched the internet and books and cant come up with
anything that looks remotely like it.
Anyone have any ideas to what this might be.
--
Regards
John Foster
g1htl.RemoveThis@yahoo.co.uk