En-nouns have four possible plural suffixes: -or, -ar, -er and - (no suffix).
Most en-nouns that end in "a" get the or-suffix in plural:
en skola, skolan, skolor, skolorna (school)
The rest of the en-nouns either get the -ar or -er suffix in plural. You have to learn which one with each word, but there are some consistancies you could try to look up on the internet.
en tallrik, tallriken, tallrikar, tallrikarna (plate)
en dam, damen, damer, damerna (dame)
Finally, en-nouns that end in -are (and some other ones) get no suffix in plural indefinite and -arna in plural definite:
en lärare, läraren, lärare, lärarna (teacher)
Actually, it's the same in Swedish. "Ankor äter fisk" would mean several fishes (since there is no article). "Ankor äter en fisk" would mean "ducks eat one fish" (Strange sentence, I know)
In this case you should definitely stick to "fisk" since the sentence implies that we are talking about ducks' source of nutrition in general. If the sentence had been "Ankorna äter fiskar" we would rather be talking about certain ducks and what they are doing right now. In this case "fiskar" works fine.
The sentence "Ankor äter fiskar" is not incorrect, but it sounds a little weird to me. "Ankor äter fisk" is a lot better.
I think it's more complicated than that in American English. The plural of "fish" is usually just "fish", except when you are talking about multiple species. Biologists talk about fishes, for instance, but in the ordinary case, you would just say fish. "I caught three fish" you might say - but not "I caught three fishes." I have nothing to add about the Swedish, however!