I tried the translation "I eat a fish" and got the error
Simple tense in English uses plural versions of nouns, so "a fish" would not work.
This is totally wrong. It's natural to say a sentence like "Every morning, I eat a bagel for breakfast" with the singular "a bagel". The only reason "I eat fish" is better here is that you wouldn't usually eat a whole fish.
I'm not sure whether "I eat a fish" should be accepted but there needs to be a better reason given for rejecting it.