Fish Lice
(Argulus sp.)

As suggested by Mike Pietrak

This creature which looks like it should be a "bad guy" in an alien movie or on X-Files is luckily only 2-4 mm long. It is a crustacean (so, related to lobsters and crabs) that acts like a louse on the skin of fish. They attach with the two things that look like suction cups, then inject anticoagulant into the fish and consume tissue and blood. It causes more problems with the hole it makes than its actual feeding, because it makes the fish suceptible to fungus. There was one freaky case of one of these attaching to the eye of a 10 year old boy, but the doctors just took it off with no harm done. (Yuck, though...) Luckily, the Western Australian Fisheries Department assures me that "two treatments with trichlorphon, a fortnight apart, will remove most or all Argulus." That puts me at ease (heh) but I'm glad I'm not a fish farmer.