"Both "pisces" and "fish" can mean a fish or many fish..." That is not correct. "Pisces" is Nom Pl = many fish, so you have to use sunt in the Latin sentence, which is totally correct and clear. The question I have, affects the English translation. The question I have, is the English translation. Does "The fish is fat" mean you have exactly one fish or could it also mean that you have many fish.
Sorry, I should add that in English, if you are talking about fish meat, it becomes non-count. So, you could conceivably have a tray of 20 fish fillets, maybe a fatty fish like salmon, and say "the fish is fat" meaning all 20 fillets. (This is true or other animal meat combos too, like chicken.)
"Poisson" is not an uncountable in French, but if you have "20 filets de poisson", you would say "du poisson". and use the singular "filets de poisson" and not the plural "filets de poissons".
It's the same in most languages I guess. Le poisson, c'est bon (literally the fish is good)= fish meat is good.
Yes, I understood that it was plural from the verb. Let's put my question this way: when would you use "penguis?"
I couldn't even find it on Wiktionary. Edit: Oops, I mistyped it in Wiktionary. It is the sing. masc. nominative as well as the sing. genitive. Got it. Thanks.