Imagine a means for fish to swim out the top of your aquarium, up to the ceiling, across the room and then down into another tank. That’s a fish highway…
The highway tube is completely sealed except for the openings at each end which are submerged below the tank water levels. Like a large drinking straw, when the air is sucked out of the tube the water rises inside it and will stay there unless you let the air back in.
(via Information Junk)