The most fair and effective counterclaim to the argument that everyone should replace meat with fish because it is a healthier choice is:
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 directly addresses the health aspects of consuming fish that were presented in the original argument, providing a valid point about the potential health risks associated with certain types of fish. The other options raise important ethical concerns, but they do not directly counter the argument about the health benefits of eating fish.