I think the top tank has a bottom, which sticks a couple of inches below the water level in the bottom tank. I watched and didn't see any fish go from the bottom tank to the top or vice versa. And the air bubbler is clearly in the top tank, not the bottom tank. Most gimmicks like this have an obvious easy "secret" as to how they work. Notice that if the top tank doesn't have a bottom, what holds up the fake plants in it? It looks nice, but maintenance on it has to be a royal pain.