Most questions that are asked of a Rav fall into the category of “issur veheter,” i.e. that which is prohibited by Jewish law and that which is permitted.  However there is another dimension called “Chamira Sacanta Meissura” there is more stringency attached to elements of health endangerment than the stringency of halachick prohibitions.  The Talmud in various places directly or indirectly uses this term in regard to various situations.

One of the most well known prohibitions in this category is the warning not to eat fish together with meat, and according to other opinions not even to eat them separately one after the other without eating and drinking in between, (Yoreh Deah 116, 2, 3)

This injunction is based on the rabbinical tradition that if fish or meat are eaten together etc. as described above there will be a health danger.  Although some other pronouncements of the Chazal in this category have not been observed through the centuries because of different times and places, the prohibition regarding fish and meat has remained strictly in place.  Consequently there is a prevailing custom of cooking fish in a parve pot in order to avoid not only the possibility of a mixture of meat and milk, but especially to avoid the element of “sakana.” 

Although attempts have been made to find some scientific or medical basis for this accepted Chumra, it should be emphasized that its observance is based on the statement of Chazal and its continued acceptance throughout the centuries by Torah observant Jews.