I found these pictures the other day of a hobbyist who has built a system of "highways" so the fish can swim from tank to tank. And you though you were obsessed!
Imagine trying to fill these with water? The logistics and physics aspect of this is mind-boggling. Better be careful though, if the water in either tank gets below the tube opening, all the water in the tube will empty out into the tanks!
The pictures were hosted on a website, but the domain has since expired. Here are the FAQ's about the highway:
What is a Fish Highway?
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. They're made of acrylic plastic, the same material used for many custom built aquariums.
Why doesn't the water spill out of the 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.
Do the fish swim in it?
The fish do swim in it.
Does it harm the fish?
As the fish swim higher the pressure drops a little but it's only slightly lower than the pressure pressure in the tank.
How do you fill it?
When the air is sucked out of the sealed tube the water from the tanks flows into it. Eventually, after all the air is removed the tube is filled with water.
How do you clean it?
Water flows through the highway continuously so the tank filters clean the water in the highway. Algea is removed using cleaning magnets for acrylic tanks.
How does water flow through the highway?
Water is pumped through a separate pipe from one tank to the other using a small pump. As the tank water levels change (one rises and the other falls) flow is induced through the highway by the force of gravity.