Good thoughts all around.
Last year I got some red comets (goldfish) and rosy red minnows, feeder fish, for the rain barrels. They got rid of mosquito larvae, as I had hoped. There was some mortality, but I ended the season with about a half dozen of each. They are living in buckets in the basement. I change their water regularly, feed them, and so forth, and plan to put them back in the rain barrels.
I have wondered about the disease concern, but do not know how to be sure, so I haven't given any to the ducks.
'Want to share, by the way, that the original minnows died but they apparently bred in the rain barrel and I have a second generation of minnows. The red comets are survivors from the original batch I purchased. For what it's worth.