congratulations on getting underway. it is a very good base system to work from.

yes the goldfish feeders have a lot of pathogens, but the biggest problem is the ammonia burn they suffer from being overcrowded. even the "fancy" pond raised goldfish have the same problems at 20 times the cost. the fancy gold fish shipped from florida to here are all breaking down with one thing or the other. just wait for the strong ones to survive and you will be fine. if you are concerned with parasites hit them with clout, formulin, dronsit, levamisol, metronidozole, flubendazole, there is a poison for just about everything.

if you buy game fish from commercial hatcheries you will probably experience the same parasites. internal parasites such as flukes, tapeworms, capillaria, flagelate protazoa, and a lot of annelides. it's easy to sacrifice one of them and look at a few slides under the microscope at about 400power is usually enough. externally you'll see fish lice, ich, parasitic cysts, anchor worms and such. it's a great study, but kinda kills the appetite.

if you wait for them to die first you won't see much but a cloudy liquid.

remember a good parasite won't kill it's host. so don't worry about it.