"אתם רואים את הדג?"

the same way.

You don’t. That’s bad English. ‘See’ is a stative verb.

It can also be used literally. As in do you see that fish - he is hot pink! Or, are you seeing the fish? (Or did I just imagine the clouds looking exactly like the fish in my tank?)

"Did" is past tense, therefor: "did you see the fish?" = ?ראיתם את הדג, and "do you see the fish?" = "אתם רואים את הדג?"

Actually this question asks whether you see the fish or not, and not about your ability to see it, so I believe "Can you see the fish" shouldn't be accepted.

We don't have 'et' in English, of course. We use it in Hebrew in front of a definite direct object. That is, a word with 'the' that a subject is doing something to. ex: Annie eats the apple -- Annie eats 'et' the apple. || 'et' comes before 'the apple' to let us know that the apple is being acted upon (being eaten) by the subject (Annie). We don't put 'et' in front of Annie because she is the subject, not direct object. We also would not put it in front of 'an apple' because 'an' (not that an equivalent word exists in Hebrew, but the concept exists, of course) denotes an indefinite [direct] object.