Because fish is singular and plural in English, this sentence could be both "Kuke au i nā i'a" or "Kuke au i ka i'a", could it not?
Why can't I "ho'o mo'a" the fish?
I think because of the errant space. Itʻs hoʻomoʻa. (Though I donʻt know if Duo accepts that.)
Yes, I put "Hoʻomoʻa au i ka iʻa" and it was accepted.