|I wondered if I could add living space to the fish tank by inverting a one-gallon jug filled with water into the tank. Would the fish swim up into it? Could this be the start of Habitrails for fish?|
As it turns out, probably not.
My fish "know" when they are about to swim "out of the water." Even when highly incentivized as they pursue food, they duck back under the tank's water level when they swim about in the plastic bottle. I've observed one fish in particular which will swim right at the perceived boundary, but if its eyes lift above the tank water line, it startles and swims out of the wide mouth of the bottle.
Apparently, the perception from the eyes overrides the perception from the mouth and skin. Fully engulfed by water, its eyes can see the boundary of the tank's water level, and this is sufficient to keep it from exploring the greater volume above.
Since the entire fish tank is "above" water, what is different about this extra volume? If I took this tank, and put it in a bigger, shallower tank, would the fish stay beneath the level of water in the larger tank?