No first hand problem with disease, but I did transfer 200 adult Golden Shiners from a local public BOW that had Eurasian Watermilfoil. I made sure that no water from that BOW entered my pond and I still had EWM show up the next year.

Maybe the GSH ate seeds and pooped them out?

BUT, any external parasites could be transferred on incoming fish, and a salt dip will cure that.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).