Technically, "Fishes" isn't incorrect:
It's just not a typical usage. I agree, though, that maybe Duolingo should discourage the usage in this case. "Fishes" in English is only used in some pretty specific cases.
As a speaker of American English, I'm going to defend "moose". We would say "What do moose drink" to refer to the general plural, and "What do the moose drink?" for the definitive plural. Same thing for deer and elk/wapiti- in American English, at least.
I totally sympathize with weirdness that is English Grammar. I think this video might help a little:
What is drinking the moose? :D "Hva er det som drikker elgene?" - would be the right question if I wanna know what is that monster, that's drinking the moose... right? And if I want to know what the moose drink/are drinking: "Hva er det som elgene drikker?"... am I right?
The animals that North Americans refer to as elk (a.k.a., wapiti) don't exist in Europe.
Eurasian elk (Alces alces alces) and moose (Alces alces americanus) are almost identical genetically; they are considered semi-species. The picture below shows caribou and reindeer as the same animal, and although they're the same species (Rangifer tarandus), it's not strictly accurate. There are multiple subspecies; caribou haven't been domesticated, whereas reindeer have.