How Do You Know Fish Is Not Happy?

One day, John and Jacey went to a park and saw fish swim in the pool, here is a dialogue between them.

John: You see? Fish are swimming in the pool comfortably without limitation, and they needn't to worry about anything, so they must be very very happy?

Jacey: You are not fish, how do know whether they are happy or not?

John: You are not me, how do you know I know whether they are happy or not?

Jacey: ???…