Is there a problem with eating fish and cheese together, such as a tuna cheese toast?
The Shulchan Aruch (YD 87:3) wrote: "...but regarding fish, there is no prohibition, even Derabanan (about eating with milk)." But he also wrote in the Beit Yosef that one shouldn't eat fish and milk anyway, because of the danger there is in doing so. Taz, Shach, Perach and Pitchei Teshuva say it's a misprint in the Beit Yosef, and it's supposed to say: "fish and meat" (with milk is permitted and there is no danger). Kenneset Hagdola and Beit Lechem Yehuda wrote that the same applies to fish and milk (and cheese), but butter is allowed, the Kaf Hachaim and Ben Ish Chai agree. The Pachad Yitzchak, who was a great doctor, was careful not to eat fish with butter too. Rav Ovadia Yosef writes that eating fish with milk or cheese should be avoided beacuse of the danger. Those who have the Minhag of eating fish with butter, may keep it. Ashkenazi Minhag is to be lenient on this issue, they have who to rely on. Each one shall keep his Minhag. Rabbi Ro'i Margalit Rabbi Dov Lior's answer