Answer: The Mishnah in Masechet Chullin (103b) states: “Any meat is forbidden to be cooked with milk, besides for the flesh of fish and grasshoppers.” Clearly then, according to the letter of the law, the prohibition of cooking fish with milk is not included in the prohibition of cooking and eating meat with milk, for the flesh of fish is not considered “meat” which the Torah has prohibited; it is not even prohibited by rabbinic enactment.
Nevertheless, Maran Ha’Bet Yosef (Yoreh De’ah, Chapter 87) writes that one should nonetheless abstain from eating fish with milk because of the danger involved. The Levush writes likewise. Similarly, the scholarly doctor, Hagaon Harav Yaakov Tzahalon, writes that one should not eat fish with milk, for this causes severe illnesses. On the other hand, many Poskim have ruled leniently on the issue of eating fish with milk. Indeed, the Rama in his work Darkei Moshe writes that he has not seen people being careful about this.
There are also those Poskim, including Maran Ha’Chida, who writes that even Maran Ha’Bet Yosef did not intend to prohibit eating fish with milk; rather, he meant to prohibit eating fish with meat. The words “fish with milk” that are written in the Bet Yosef are a typographical error. Thus, although in general we rule in accordance with the opinion of Maran Ha’Bet Yosef, nevertheless, one may indeed act leniently in this case.
Based on this, Hagaon Harav Shalom Messas zt”l issued a completely lenient ruling on this matter since it is unclear whether or not Maran Ha’Shulchan Aruch prohibited eating fish with milk in the first place in addition to the fact that several Sephardic communities customarily acted leniently regarding this matter and ate fish with milk and that doctors in our day and age agree that there is no danger of contracting any severe illnesses by eating fish with milk.
However, Maran Rabbeinu Ovadia Yosef zt”l writes that the Sephardic custom was to indeed prohibit eating fish with milk because of the danger involved; indeed, several Poskim write clearly that the Sephardic custom was to rule stringently on this matter. It is quite difficult to believe that this was a typographical error in the Bet Yosef, for this custom to prohibit fish with milk would not have spread with the agreement of the greatest sages of several generations were the very basis of this law to be a mistake which stems from a typographical error. Certainly, if this would be a mistake, the greatest sages in the generation of Maran and the following generations would publicize their opinion that there is a mistake in the Sefer Bet Yosef. This is especially true since Maran himself edited his book and did not raise any issue with these words. Based on this custom, several great sages, including Hagaon Rabbeinu Yosef Haim in his Ben Ish Hai, have ruled stringently regarding this matter. It is therefore preferable to act stringently and abstain from eating fish with milk. However, if fish was mistakenly cooked with milk, even Sephardic Jews who customarily abstain from eating this may in fact act leniently and consume it.
Nevertheless, regarding eating fish with butter (which is not considered like actual milk as opposed to other dairy products), several Poskim have ruled leniently on this matter, even according to the Sephardic custom. Indeed, Hagaon Harav Abdullah Somech zt”l of Baghdad writes in his Sefer Zivchei Tzedek that the custom in his city was to permit frying fish in butter. Thus, those who customarily eat fish with butter may continue to observe this custom. Ashkenazi Jews customarily rule leniently on all of the above (including eating fish with milk or cheese) and they may indeed continue with their custom as well.