The most fair and effective counterclaim to the argument that everyone should replace meat with fish because fish is a healthier choice is:
Large fish such as grouper and bluefin tuna are known to have elevated levels of mercury, a toxic metal that can cause a wide range of health problems.
This counterclaim effectively addresses the health aspects of eating fish, which is the primary argument presented in the passage. It introduces a legitimate concern regarding the safety of consuming certain types of fish while also maintaining a focus on health, making it a relevant counterclaim.