I have separate grates for fish and meat. I close the lid when I grill. Can I eat dairy after I eat the fish or do I have to wait a certain amount of time because I close the lid of the grill when I grill meat?
You do not have to wait after eating the fish, even though the fish was grilled in a meaty grill. The reason is that we don’t have to wait after eating food cooked in a meaty pot because it only has the absorbed taste of the meat. However I am assuming that you make sure to keep the lid clean, and clean it between uses from meat to fish etc. Otherwise you will be grilling the meat in a pot that has fish residue in the lid, and it is mixing with the steam of the meat while it is grilling, which is problematic.
Y:D 116 -2 Chasam Sofer ibid-2, Halacha Berura O:CH 173-9, Shevet Halevi 1-111(4).