"Who eats fish?"
Translation:¿Quién come pescado?
Quien = singular Come = singular Quienes = plural Comen = plural
If you use singular you must stay consistent through your sentence and same applies when using plural. Quien comen = wrong but Quienes comen = right and vise versa.
In terms of fish (pescado) I know that in English you use 'fish' for both singular and plural regardless of how many fish you are referring to. I am a native English speaker and notice many children and adults making the mistake of referring to multiple fish as "fishs" or "fishes".
I could be wrong but for that reason I'm guessing pescados is an incorrect word too .
'Pescados' is a correct word. In English there is no separate word for the plural form of 'fish' doesn't mean it would be the same in Spanish. Don't compare any two languages, every language has its own rule and you have to learn it by that way.
'Pescados' is the plural form of 'pescado'
It seems to me like the difference would be, for example, between A. knowing that one of your kids eats fish but you aren't sure which one so are trying to figure out, "quién come pescado" versus B. in a crowd asking "okay, who all here eats fish?" which would use the plural option.