Hi if I used a meat knife that I did not use in a month to cut lemon that I am putting with fish is it ok?
It is forbidden to eat fish and meat together because of sakana, danger to one’s health (שו”ע יו”ד סי’ קטו סעיף ב). This prohibition only applies to eating fish and meat that were cooked together or otherwise mixed. The taste of meat that was absorbed in a utensil is not considered dangerous when eaten with fish, or vice versa (ט”ז סי’ צה ס”ק ג, כף החיים סי’ קטז סעיף כ). Therefore, it is permitted to use a meat knife to cut something sharp, like a lemon, even though it will be eaten with fish.
All the best