How is this done?

[sphelps]

Originally Posted by lastlight

if you placed a mostly full pop bottle of water into the sink upsidedown quickly, the water would stay in the bottle. Could you not then have an air pump with the input inside the bottle drawing from that air cavity at the top? Blow this air into the bottom of the bottle so it just goes right back where it came from?

Yeah that's what I was thinking but you would need one hell of an air pump. That tank looks 4 feet tall, that's a pressure of 4 feet of H20 the pump would have to overcome. And I think it would have to be more along the lines of a venturi as an air pump wouldn't last too long with the moisture and humidity it would be exposed to in this situation.

[sphelps]

I guy could probably use a float switch to activate some kind of vacuum system when the air pocket got to a certain size.

[ElGuappo]

If you look at the top of the tank he has a line drawn at the fill point of the waterline.

I also think this is done wiht mirrors. one scene the fish are in the top half then it cuts and they are all in the bottom. then when they swim under the top part they disappear. you can clearly se the top has a bottom pane of glass.

[The Codfather]

http://www.youtube.com/watch?v=Z7tq4LRurMs
This shows how the tank was built. No tricks, this works.