"It is mainly fish."
Translation:Es principalmente pescado.
'Es fandetalmente pescado' wasn't accepted, does anyone know why was I wrong? ")
My first guess would be because "fandetalmente" is not a word. If that was just an error here and you wrote es fundamentalmente pecado as a response, then the reason it was wrong is that fundamentalmente indicates something of essence, that the dish is essentially fish, not that a primary ingredient is fish.
Why is "pescados" incorrect? "fish" in English can be either singular or plural obviously so I didn't know.
The problem with your thinking is illustrated as follows: I have one fish, while you have two fishes. In a net full of fishes we have a fine catch of fish. In Spanish the same logic applies, so long as the noun is uncounted it is singular.
El plato es principalmente una mezcla de dos pescados
lily130472 tiene razón. I have one fish, while you have two fish. Fishes significa dos o más tipos de pescado y no es muy común.
You have only replied to part of the response, and it was the part that only pointed out that there is a plural of "fish".
To address the real point: sure, a fish plate can be composed of many fish, or just one. That "fish" can be collective is irrelevant though because this example phrase is uncounted, and is therefore going to be singular. That rule is valid in both languages. To plural the Spanish version is to add a meaning that is not there in the English.
En inglés se dice "It is mainly beans". ¿Es "Es principalmente frijoles" incorrecto?
in hover it says mainly is also fundamentalmente es fundamentalmente pescado should be accepted
What about "Es principalmente peces"? I'm guessing peces means fish that are alive, but how would you answer "What's swimming in that pond?"
Absolutely for your situation your translation would work. Duo does tend to be myopic when they have a certain idea in mind and doesn't see other possibilities. But since the program works better when you limit the possible correct answers, I favor giving them one or two of these when you concede that their meaning is the more likely translation. I have noticed that Duo has gone more and more to the building block answers to avoid having to accept too many answers, but I personally prefer having to construct the sentence myself, so I have become much more tolerant of a limited translation if it is predictable. Only when the user can't reasonably guess what Duo wants (like which form of you) do I file for a correction. But the choice is yours.
By the way, do I want to know what ELSE is swimming in that pond?
"It's mostly fish" seems like a smart (in a bad way) answer if somebody's asking "What's swimming in that pond?" which they ask because they see something like a rodent or bird swimming across that they're not accustomed to.