Without water parameters there isn't much anyone can do for you. We have no way to begin to trace the source of the problem, which is needed to resolve the problem... and there is no way to know if the water conditions are safe enough for medication.
I would strongly urge you to get yourself an API master test kit which contains the needed tests. They can usually be found for less than $30 for the entire set, so they're not overly expensive. Once you have the results, post the exact numbers here and then we can discuss diagnosis and treatment.
What I am seeing in the photos could be one of a few different things. Its important to know for sure what you're dealing with when medicating a fish, if you wish to do so safely. The water params will either confirm or eliminate some of those things.
Also, more information about the tank/set up is needed. What is your maintenance routine? (water changes, how often/how much each time?) Whats the water temp? Are you using water conditioner?
I know your guess is that feeder goldfish caused this, but that is not necessarily accurate. At this point I would call it a "potential" cause at very most, especially since not all of your fish are affected.
Sorry I can't help more at this time, but I will keep watch for those water params.