No doubt disease is an issue to worry about. I am confident in my ability to identify fish species so as to avoid adding unwanted species. If you are not comfortable in fish identification, this may not be an option for you.
With that said, just because a fish comes from a hatchery doesn't mean it isn't diseased... It may reduce the risk, but is no guarantee. And many hatcheries have led to the introduction of unwanted species into a pond. I can't tell you how many times I have seen GSF or goldfish mixed in with GSH or FHM. Or bullheads mixed in with CC.
