Robyn,
My granddaughter found a mussel. My daughter put it in her pond. Is it safe to be with Koi and Goldfish? Do they cause disease or carry parasites? I appreciate your help!!
Ellen

The mussel won't directly hurt the fish but the fish could hurt it (try to eat it when it opens). Mussels/clams are intermediate hosts for some fish parasites so it could introduce something to the pond.

It will only cause problems if you have bitterling. young mussels drink bitterling blood

Not exactly I don't think. Bitterling require mussels/clams to spawn. If the mussels/clams themselves spawn, the babies may live in the gills of fish, any fish. In small numbers, this doesn't hurt the fish really but if you had a ton of mussels/clams that sent out spawn, it could be harmful. See http://www.ncwildlife.org/pg07_WildlifeSpeciesCon/pg7b1a.htm
http://www.ivb.cz/fish/reichard_research.html