2 possible and accepted solutions here:
everytime the same problem. sometimes fish should be singular, other times plural is meant
There is no way to know if they want one or more fish, so why is using the singular wrong?
again the same problem: when Duo wants the singular FISH translated as fish or as fishES?
Piscem salsum velim. Unus, ille est.
No indication in English if singular or plural FISH
Duo firmly, and despite evidence to the contrary, believes the word "fishes" does not exist.
I wish to repeat the comment made by Perce Neige (immediately below) a month ago.
Why plural, in English it was Fish, not fishes.
So piscis salsus is correct in the singular!
Piscem salsum velim would be the singular version of the sentence they give.