The most fair and effective counterclaim to the argument that everyone should replace meat with fish because it is a healthier choice is:
D. Some large fish such as grouper and bluefin tuna are known to have elevated levels of mercury, and consuming them in excessive amounts can cause a wide range of health problems.
This counterclaim provides a specific health concern related to fish consumption, which directly challenges the idea that fish is an unequivocally healthier choice. It presents a nuanced view by acknowledging that while fish can be healthy, certain types can also pose health risks, making it a well-rounded counterpoint. Additionally, it does not dismiss the value of fish entirely but rather highlights important considerations for consumers.