Right. Imagine being watched by humans with no way to hide in the tank. No matter where you go, someone can observe you and you won't even know. They will alter your behaviour by placing new products inside the tank every so and then. Put new shiny stones, hanging weed etc to coax you to do something they want. You may not get a big tank because your owner might not be able to afford the bigger tank as price of them keep increasing. Pretty isolating.

It's good that you are not a fish but a human.

Would you be? Depends on:

  1. Were you born in the fish bowl, or did someone place you in it after you grew up?

  2. If the former, were you ever given a chance to leave the fishbowl and experience different life?

My hypothesis is that living things can't be depressed about stuff they are unaware of, since depression is a relative state rather than an absolute state.

Isn't the Earth sort of a people tank, though?

It's pretty big, but there are only so many variations on "tree", "mountain", "valley", "cave", "swimmy thing", "flying thing", "walking thing", etc etc.

It's not like you can ever expect to walk over the next hill and see an octo-monkey spiraling across the veldt.