I had probably 1,000 -1,500 Fathead minnows die due to thier digestive system being blocked by these parasites. This was in an Aqua-Culture class in High School ('98-'99). I noticed after about a week or so they stopped feeding and several died. Then, after about a month I only had a handful left. During this time the head Fisheries Bioligist for West Virginia visited our school. She informed me that I was doing something wrong to have all these fish die. I pointed out the parasites on the outside of the fish and told her that I think those are problem. After she told me I was wrong again and, that they only live on the outside of the fish, I proceeded to cut open a bloated 4in. minnow and pull out hundreds of these tiny creatures. She had never in her career seen anything like that.