In that sentence and lot of others an indefinite article can pop up absolutely randomly. Sometimes you mark it as a mistake. In next exercise, when you translating the sentence as it was suggested previously by your system, you also mark it as a mistake, but now you suggest that the first suggestion, that wasn't count is right!
The indefinite article doesn't "pop up randomly". It is required because "Fish is coming" is not a valid English sentence. Hebrew doesn't have an indefinite article so you have to add it where needed when translating to English.
Why can't it be "Fish (as in the food) is coming?" That's a valid English sentence.
I got this as a spoken sentence to translate. Other than trying to make more sense, is there any way to know that this was not דג בה, "a fish is in it"?
Not really.. And anyway, דג בה actually wouldn't make any sense as standalone sentence and I don't believe it would ever appear like that in Duolingo
I agree with the commenter several below. Fish is coming is correct if used in terms of food. Since so many of the comments are food related (i.e. meat and cheese, beef, wine, etc.) this should be a proper solution and perhaps the only correct solution, but certainly not incorrect.
I would say that this fish is definitely A FOOD, it is a "food" lesson, not "animal" lesson. So, it has to be without the article. Bon appetit!