Same Pot for Meat and Fish
Is there an issue with using a meaty pot to cook fish? I know meat and fish should not be cooked or eaten together.
The Aish Rabbi Replies
According to the letter of the law it is fine to use a clean meat pot to cook fish (since kosher tastes (as opposed to non-kosher tastes) absorbed inside metal are considered insignificant). However, many (especially those from an Austrian background) have the custom to use separate pots, out of concern that that taste might be transmitted by the cooking. (Sources: Shulchan Aruch Y.D. 116:2, Shach 2, Maharil HaChadashot 86, Avnei Nezer Y.D. 85, Yabia Omer IV Y.D. 4.)
See this past response for a longer discussion of the practice not to eat meat and fish together.