How is this done?

[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.