Yes chickens can eat fish. I watched this one nature of the chicken documentary one day and some people even feed them big macs.. I thought it was strange but the chicken was fine. Chickens are like pigs, they eat pretty much everything. My chickens have ate fish mainly because of my ducks and they've been completely fine. It's been years.
I also have ducks and chickens living together, well, only 1 chicken left since my other one drowned, and they were raised eating little minnows that came from Gander Mountain.
They ,in turn, get theirs from a farm raised source.
The ducks and their chicken are all over 1.5 years old and happy and healthy as can be.