Why not "Fish swims every day?"
What jfbarnes21 said. "Fish swims every day" doesn't make grammatical sense.
You need an article at the start of your sentence, and the Danish sentence has none so must be referring to fish in the plural.
Fish gonna have bikini body ready soon
As an analogy, you could say: "Sharks swim every day." or "Sardines swim every day." I think that the confusion here lies in the fact that the plural of "fish" is "fish".