Diseases are much more prevalent in an aquarium setting than in a pond. A typical forage pond will have hundreds to thousands of times the volume of the aquarium, diluting possible pathogens, which are always present in some quantity, however minute. Typically, bait shops get their fatheads from the same minnow farms that we buy them from, and only hold them a few to several days.
In my little test, the RRs were used as a control. They were healthy living in the aquarium. The FHM brought whatever killed them with them. (I only added the FHM to the tank, not the baitshop water.) I suspect, in most cases, you are correct about the baitshop fish coming from the same place as those we buy to stock our ponds. That tends to make me suspect either the baitshop supply truck tanks or the baitshop tanks as the source of the contamination of the FHM. The subsequent death of the RRs later on suggests it is contagious but takes a few days to develop and kill the fish.
An interesting second test might be to sterilize the aquarium, reset with new RRs and then add FHM direct from the fish farm.
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