"Ducks eat fish."
Translation:Ankor äter fisk.
I had thought fisk was an -en word, meaning the plural would be "fiskar", correct? Or can fisk be both -en and -ett, like kaffe, öl, etc...
Correct. The plural of fisk is fiskar. However in this case it is that you actually say that they eat a group of food, in which case the singular could be used. Compare with the following:
"I'm a vegetarian, but I eat chicken." - "Jag är vegetarian men äter kyckling."
It's really in 'Plurals', but there's a bug (or possibly A/B test) at the moment which causes sentences to show up in later skills sometimes.