He must have meant Kuf Ayin Gimmel (173:2). There the Mechanber says, “Between meat and fish, it’s an obligation to wash because it is difficult for something else, and by danger we are even more strict than Issur.”
The Mishna Brura(3) says that “difficult for something else” means eating fish and meat together causes leprosy. However he says to see the Magen Avraham who says that it is possible that nowadays there is no danger. This is because nature has changed.
In other words, it was dangerous at one time, but may not be anymore. Nonetheless, Moshiach Agent has a valid point. The MINHAG amongst everyone I know who keeps Shabbos is not to have fish and meat on the table at the same time. If you want to feel like you are a member of the group of people who keeps Shabbos, then you will need to conduct yourself according to this minhag. If you have an uncontrollable desire to put anchovies on Chicken Ceasar Salad, don’t tell anyone about it except for your local Orthodox Rabbi 🙂