This is actually quite annoying issue in airtravel, there are ample about of made-up diets and imaginary problems but for those with allergies towards seafood there is never a pre-picked category. One can, and I do when travelling in C, call CS and ask for fish-free menu. I reckon globally fish related allergies are quite rare and no fish-alllergic would blow up the plane or create general somefeud on getting wrong food so in generally naturally airlines have to draw the line somewhere on how many categories to add up in the dietary options.