By Rabbi Eli Gersten
The Gemara Pesachim (76b) teaches that one may not cook fish and meat together since this combination is considered a sakana. Shulchan Aruch (Y.D. 116:2-3) adds that one may not even eat meat after fish or fish after meat unless one eats and drinks in between1. Rama adds that one should not cook open meat and fish in the same oven because of raicha (aroma), though bidieved we say that raicha lav milsa. Magen Avrohom (O.C. 173:1) questions whether this sakana still exists today, however the minhag is still to be machmir.