For me the biggest advantage of fish keeping over caring for a dog, is with fishes you do what you have to do on your own time. I work a lot and with the family there is always lots going on. Our dog needs to pee, needs to eat, needs to walk, etc so or schedule always has to take that into account.
The fish tank ? No worries, we went out of town one week without even bothering with an auto feeder... Came back everything was fine.
The dog? I had to beg coworkers to give him a roof for that week...