I've never heard that fish effect the eggs...I guess someone with a world class palet might notice something...but I've never heard of anything like that. Ducks are vegetarians for the most part BUT they will eat minnows & bugs out of necessity. The minnows are more fun for the ducks...they love to watch their food swimming around for a while. Mine weren't sure what to do with minnows at first. They looked at the fish, looked back at me and I walked away thinking I was going to have to throw the fish in the pond...5 minutes later..no fish! Just a couple ducks wanting to take a nap.
I can;t see how that could be a problem unless they ate fish everyday, and a LOT of fish or only fish. I wouldn't worry too much over this.
Things like minnows, certain treats and other foods that ducks will eat, does not mean they are supposed to have it everyday. Once a week for fish and they're fine, I would think.