Est-ce que tu aimes les poissons?
This was marked wrong. Est-ce que tu aimes bien les poissons? WAS CORRECT. CAN SOMEONE EXPLAIN THIS PLEASE?
It should be right.
"Aimer" has a whole range of intensity in its meaning, from "like" to "love". It depends on the context. Some will say that "aimer" < "aimer bien" but i disagree, because it is not always the case. For exemple, when talking to someone, "Je t'aime bien" could mean, "I really like you", while "Je t'aime" could mean "I love you".
Also, I've rarely said "J'aime bien". I'd rather say "J'aime beaucoup" or "J'aime vraiment". I think "J'aime vraiment" is better most of the time, since "beaucoup" invokes a quantity while "vraiment" invokes an intensity. I must point out that "J'aime bien" is not as strong as "J'aime vraiment".
In the case of the fish, I think we could assume that "aime" < "aime bien" < "aime vraiment"
PS: I'm from Québec, it might be different a little bit in other parts of the world