"Rosa has fish."
Translation:عِنْد روزا سَمَك.
Never heard of سِماك (simák) - it's either سمك (samak) or أسماك (asmák). (samak) is collective indeed, and hence it is considered a plural. In Arabic, one word can have more than one plural: e.g. شهر (šahr:month) can be put into plural as شهور (šuhúr) and أشهر (ašhur); or like شجرة (šajarah: tree) can be in plural as شجرات (šajarát) or أشجار (ašjár).
However, those who are deep into the literature of the language and into the linguistic details of Arabic would sometimes signify that some specifics plurals are used for a certain number. In the example of شهر (month), the plural أشهر is used mostly if the time period under discussion does not exceed 3 months. Anymore than that then the plural شهور is used. Anyway, these are very fine details and not quite required for learners nor the public is definitely aware of such differences, but any of them are understood nevertheless.
Thank you for your explanation. You will find سِماك with the other plural اسماك in the dictionary, e.g. Hans Wehr 4th ed., page 503.
Edith remarks on the original question: I guess thomanice as I were triggered with a lot of examples with nouns in singular. Changing to a collective noun was not expected. "Rosa has fish (coll)" <--> "عند روزا سمك" "Rosa has a fish <--> "عند روزا سمكة"