Marlin from Finding Nemo is insane and trying to find his son who doesn’t exist because Nemo in Latin means no-one and when we see Nemo interacting with the fish it is all in marlins head that’s why the little girl is a fish killer.  The fact that we see the fish at the end only shows that someone let the fish from the dentist go as a way of showing that marlin gives up on Nemo and why he makes the new joke is because he comes out of his depression and finally decides to let it go.