A "fish orange" would be an orange that tastes like a fish, because in English the adjective comes before the noun, making "fish" an adjective and "orange" a noun. Or it could be an orange that only fish can eat. Or an orange made out of fish. Or an orange that lures fish to it by imitating an orange for fish to eat but is in fact a fish-"eating" orange, like a venous fly trap.
...because they are hungry?
Many people eat carp in Europe and Asia. Goldfish are just small carp. Personally I have eaten carp and found that there are too many little bones for my liking :L
Anyway, I looked up "goldfish" and the French name seems to be "poisson rouge". So maybe we are all blowing bubbles in this thread.
No. DL tries to maintain high standards of language and because there are people learning French while also learning English, allowing an error like that would lead to careless mastery of the language. DL had no way of knowing if it was really an accident or you really don't know you are supposed to use "an". It would be just as wrong for someone to make the careless mistake of writing un in haste when the person meant une, or poison when s/he meant poisson. DL being this strict forces us to be meticulous, which I think is a good habit to have.
When I was asked to pick answers, "I am eating an orange fish," wasn't an option. I had to choose, "I eat an orange fish." Why would, "I eat orange fish," not be an acceptable answer in this case? 'I eat an orange fish' does not sound right to me. In English we would say, 'I am eating an orange fish,' 'I eat orange fish,' or 'I eat some orange fish.'
"I eat orange fish" is an incorrect translation, because the original French clearly indicates "an" orange fish: if the original French uses an indefinite article, then the English translation should use an indefinite article. ("I eat orange fish" is a perfectly fine sentence in English, but it does not mean quite the same thing that "I eat an orange fish" means.)
Why does "I eat an orange fish" not sound right to you? "I play a silver saxophone," "I kick a red ball," "I drive a blue car." Those sentences are all fine in English, and "I eat an orange fish" is an acceptable translation of "Je mange un poisson orange" (as is "I am eating an orange fish").
Yes, in English, "I am eating an orange fish," "I eat orange fish," or "I eat some orange fish," are all perfectly fine sentences, but only the first of those sentences correctly translates "je mange un poisson orange" (and "I eat an orange fish" is another possibility):"
Je mange un poisson orange. = I eat an orange fish.
Je mange un poisson orange. = I am eating an orange fish.