What stumbles me a bit by reading through your post is the fact that cows use the livestock tank to drink (Ridicilous, I know, thats what its supposed to be for

). Could the cows carry something into the container that isn't good for fish, some sort of parasites? It's probably completely normal and harmless to the cows, but could be a problem for the goldfish.
I think its an internal problem as well, and either bacterial or parasitic.
There is a product called Pepcofood or PetcoFood, something like that. It comes in a little container and can be fed to treat internal parasites. Its a rather fine and small food, and easily digestible for the fish.
As for internal bacterial problems, you can order Medigold from a place called Goldfishconnection.com. Its a medicated food and takes care of a wide range of bacterial problems....