What is Darwin on the amazing world of gumball?

Yes. Darwin is a fish. The clues that led to the realization that he was a fish is:

1: In one episode Darwin woke up from a fishbowl

2: If you remove his legs and arms, you have a perfect replica of a fish

3: The thing sticking out of Darwin's head is a fin if you look closely

4: And if you notice, when Darwin is not talking his lips are always puckered up like a fish.

Fin + sleep in fishbowl + Head of a fish = Darwin/Goldfish