I believe the clear PVC that you can use is resistant to growing algae. I may be wrong though. If my 55 and 20 were the same height I'd do this bridge that showed in the link. That one picture...I would never imagine a fish swim all the way through it. Someone posted stuff about having cat ladders and those water bridges...I've seen houses that had those "cat ladders" except with trains running all through the house. That would be tight too.
My favorite lfs in Cypress california had a beautiful tank set up like that 55 gal acrylic tanks set up on either side of the fish room door with a big acrylic tube running over the top. and, yes the fish used it all the time I spent hours in there just watching that tank!
but that was many moons ago when I was about 17/18 and had a 20 gal on the floor of my room. mom had the 30 in the living room and an old 5 with a metal frame on the kitchen bar
I like the bridge concept, but don't understand why the guy in the pic that matt posted would spend all that time and money on the bridge, and have it going between two what looks like 20 gallon tanks???
I think this also incorporates the same theory as the inverted aquarium, just a matter of size and dimension of the application. But this thread and especially the link on how to do it has convinced me to build one between my two 80 tanks which will only be 5 inches apart.