As far as contaminating the fish I don' t think ducks are going to mess them up. Fish get " poisoned" from " breathing" poisoned water. When there are chemicals and hard metals dissolved in the water they absorb those chemicals when they filter the water through their gills. Kind of like eatin old paint chips and getting lead poison. So if the duck refuse isn' t getting mixed with the water badly and the fish aren' t consuming it I don' t see how it' d effect them. If it' s a concern have someone test it to see if it' s safe, but if you see any fish coming out of there with legs and four eyes you may want to throw them back.

Brandan