Mr. Baird, (Yes Coach and I have the same last name)
I have an excellent text I can peruse when I get time but a better pic would be nice. However the exact species is academic. This being an external parasite it's easy to get rid of in tank fish. (If it truly is attached to the outside of the fin.) Is it possible the bluegill picked up the parasite in the tank or did it have it on when you put them in?
As far as your fish in the pond being inundated by this parasite (if it came in from the pond) it's doubtful and a treatment for the entire pond is probably not practical. You will probably see a few fish with the parasites, but it's most likely a seasonal thing, or if something is stressing them in the pond. I've seen some of my spawning bluegills attacked by parasites, but the rest of the year they are clean as a whistle.
It could be the parasite will be here one day and you'll never see it again too.
Fish can handle parasites to some extent and be otherwise healthy and grow normally.
As far as treating the fish in the tank it's easy to do that with ordinary uniodized table salt and dip treatment. Here's what you do:
To some tank water and mix 4 oz. of uniodized salt per gallon in a bucket or similar container. Make sure it's dissolved. You now have a 3 percent solution of salt the same as typical salt water.
Net the fish and submerge them in the saltwater solution one at a time for about 20 seconds or until the fish turns on it's side. Then immediately put the fish back into the tank. If the fish are in good shape they should be able to handle this fine. The saltwater will cause the parasite to implode and effectively kill it.
If possible, keeping your tank water at 0.2 percent salinity indefinitely will not only be less stressful on your fish, but should keep external parasites in check.
I do this dip treatment whenever I bring fish in from my ponds to my Recirculating Aquaculture System. I have no problems with parasites in my tanks. All fish in the tanks are healthy and clean of parasites.
If for some reason you first fish has problems with the dip treatment you may want to reduce the salinity or go with a bath treatment of lower intensity.
If you want some excellent reading on the subject of salt treatments and parasites here is a great piece by Dr. Kebus of the Wisconsin's ADDL. It basicially is written for Koi but applies to the fish also. Not too academic to bore the shorts off of you. Written for us laymen.
http://www.aquarticles.com/articles/ponds/Kebus_Salt_Treatments.html
