On reddit there is who says that you should use the word ´yhden´. But even there is not so clear.
¨The boy eats an apple. ~ Se poika syö yhden omenan.¨
That's optional. The accusative case already implies "yhden", so you could omit it and the meaning would remain entirely unchanged and it would also remain grammatically correct. In the translation of "a fish", you could use "yksi" instead because it's the nominative case form, but the nominative case already implies that it's one entire thing, so it's entirely optional as well.
Edit: And by "imply" I mean "indicate". There's no ambiguity about it.