A unique principle of physics allows you to suspend a container of water upside down inside of another container of water. I know that sounds a little bit complicated, but have you ever blocked the top of a straw with your finger, then pulled the straw out of the drink. Then you know what i'm talking about. You can do the same thing with your aquarium.
You will need your aquarium and any container that is closed on one end that will fit inside your aquarium. The containers rim must be strong enough to support its weight plus the weight of the water inside of it. Now support that container in the tank, making sure the supports are strong enough to hold the combined weight. A few decorative blocks work well as supports. Next get a piece of air tubing and put it through a piece of foam so the tubing can float to the top of the second tank. Now you need to suck the air out of the top container and watch the water level rise in the second container. Keep an eye on the water level in the main tank ensuring that the water level doesn't drop below the rim of the top tank.
These aquariums are truly unique, make sure that you watch the water levels closely. If the water level in the lower tank ever falls below the bottom of the top tank, air will get into the top tank. If you allow air into the top tank it would be like removing your finger from the straw. Depending on the size of the top tank you could have water to clean up.
I have also seen this done to koi ponds. It allows the fish to see another world.
