More fish perception experimenting!
Following up on my idea to test whether it was familiarity with their habituated "wet space" or if they were following what their eyes informed them, I wanted to test if an external water level would change the fish's behavior. I decide to start very small, rather than go buy a larger tank to set the smaller tank in (Using a larger tank, I could create a surrounding volume, fill it halfway with water, and create a false "top" boundary for the fish to see).
Using a one-gallon pickle jar, I filled it to half as high as the real tank's water level, and set it next to the fish tank. As you can see in the picture, that was enough to convince the fish to swim in the lower half of the tank. Their behavior was quite interesting. They would swim in the entire column of water unitl they got within about 3 body lengths of the outside jar; then they would startle, and swim beneath the level of the water in the jar.
The startle reflex suggests that they could become habituated to swimming "above water." It also makes me wonder about why a fish would even know what that boundary looks like "from above," and why it makes them swim below it.