The guy's description says it is some sort of vacuum, but if that is the case, wouldn't it be pulling all the fish as well as any shrimp you are trying to feed to the fish back into the vacuum? It's really cool either way.
The tank works through pretty simple physics - water sinks in air, and air rises through water. Since the holes in the tank are below water, air cannot enter them (we must assume the rest of the tank is air- and water-tight). You can do the same thing at home with a pint glass and a sink full of water. Let all the air out of the glass, and pull it upside down out of the water - you'll pull water up in it until the lip of the glass breaks the surface of the water in the sink.