"We throw the fish to you."
Translation:Piscem vobis iacimus.
I think "ad" + accusative would have been better: it can be a place complement (the fish going toward you) and even an ostile movement (the fish going against you)
A general statement as in "I like fish" would be a plural..."Pisces mihi placent." The singular may be determined if the indefinite article "a" can be put in front of fish: "I'd like (a) fish." But if you did this to the "I like (a) fish" example, you'd be stating your affection for one specific fish. This Duolingo sentence could have a singular or plural "fish", although someone throwing many fish seems excessive.