Q&A from the OK Consumer Hotline

Q Can fish and meat be cooked one after another on the same grill?
A The Shulchan Oruch, Yoreh Deah 116, prohibits cooking or eating fish and meat together. While some have the minhag to use separate keilim for fish and meat, many do not.

However, even those who do not use separate keilim need to make sure that the utensils are thoroughly cleaned between uses. Given the difficulty in completely cleaning a grill grate, the  OK does not recommend using the same grate for fish and meat. A second grate, designated for fish, should be purchased if one uses their grill for both fish and meat.

If one wants to switch permanently from fish to meat, or vice versa, the grate should be thoroughly cleaned. Kashering is not required.