What does it mean if my chicken tastes like fish?
I can think of several things, but it's not the definitive answer (not enough info in the question):


Oven was used recently for baking fish
Dish/pot was used recently for baking fish
Chickens were fed food containing fish. I don't know if this still happens, but in my country (The Netherlands) some 30 years ago, you could buy a 'powdery' chicken food that was some mix of ground-down grains, corn, and fish serving as a cheap protein source. You could definitely taste that in the eggs - I don't remember about the chicken meat itself.Note that chicken meat is sort of 'lowest on the rung' where quality is concerned, and maybe this was a batch from someone looking for a quick profit.


Note that the smell of fish permeates/overpowers quickly and that taste is very much determined by smell.