Back in 2009 I worked as a Pet Care Specialist at Petsmart (unfortunately!), and I had to deal with a lot of fish parasites and other maladies the fish shipments arrived with. While looking at the feeder goldfish tank, I saw a curious green spot on a comet's tail. I took the fish out to further inspect, and the green dot fell off the fish! It was a fish louse. Nasty little things! Here's my detailed photo and a video that shows its size in relation to my finger. I'm glad I've never had these in my own aquariums at home!
Fish lice are one of the few fish parasites that can be seen with the naked eye, and they pose a serious threat to aquarium fish. They use a sting to insert digestive enzymes into the fish, and then feed on the liquidized body fluids. They cause tissue damage at the site, and can also create openings for opportunistic bacterial infections. Fish lice should be picked off gently with a tweezers, and the tank should be treated using Dimilin according to the directions on the package.