"Fish live in the streams."
Translation:Les poissons vivent dans les ruisseaux.
Any statement of generality, whether true or not, uses the definite article*. If I want to say "fish live on land", I would still use the definite: "Les poissons vivent sur terre." Obviously, that's not true, but I'd be correct grammatically. Or if you want a semi-true statement, "Les poissons vivent dans la mer." Still the definite article.
If you were to exclude things and say "some fish live...", it's better to quantify it with "certains" or "quelques" as in "certains poissons vivent..." or "quelques poissons vivent..." It's good to keep in mind that "des" is simply the plural of "un" or "une." Using "des" in the subject means "more than one".
*The asterisk above is because something like "un poisson n'a pas de jambes" can also be considered a statement of generality. Using a singular indefinite article, of course.
“Des poissons habitent dans les ruisseaux” should be accepted because this sentence can also mean that a few fish live in the streams aside from the general statement that fish live in streams.