Leach(?) problem!

So a few weeks ago, I acquired 11 Rainbow Darters. They made it safely home after a 6 hour drive, and survived the first few days without any problems at all. They were eating frozen bloodworms, and even picking at flakes. Three of the darters, however, had what I think were leaches. I found a topic on here (http://forum.nanfa.o...ge__hl__leaches) and did the tweezers with one fish, the alcohol with one, and left one be. The two I pulled out both died within 24 hours. The third, that I left the "leach" on is fine. After another 2 weeks, it fell off. I thought the problem was solved, but yesterday I noticed I now have quite a few fish with the "leaches" attached, so I need to do something, but I won't repeat my last mistakes.

Now, let me describe these "leaches." First off, they're tiny. The biggest was maybe 2mm long, the shortest less than 1mm. They're a greenish color, and when I touched them they got short and fat. I've only seen them on the fish's fins, not on they're gills or scales, but that doesn't mean they aren't they're. Darters aren't exactly easy to examine...

All of the fish seem to be doing fine, so it doesn't seem to be an emergency, and isolating is not an option, since I am fairly certain that these creatures have permeated my tank as a whole at this point.

Also, my uncle mentioned a chemical he used to dip his salt water fish in prior to putting them in tanks to kill/remove parasites in the 1970s, but he couldn't remember for sure what this chemical was. (Maybe potassium manganese?) Any thoughts on this would be helpful to avoid this in the future.