The ten year old really got me stumped on this one...
Hey CtC, Phoenix. Cheers for giving us a good excuse to give the old physics skills a workout =)
.. is to get a bubble into the tank. What forces act on it? We have a column of water pushing down (height of water times diameter of bubble minus hemisphere of bubble in tank) and nothing pushing up. The force we require to push this up is equal to the weight of water pushing down. This is provided by the buoyancy of each bubble pulling on the string above it and is equal to the volume of water the bubbles displace. So the total volume of the bubbles above the entering bubble must be slightly greater than the total volume of the column of water directly over this bubble. ie the tube mentioned by blue beta is the closest we can get to this with all forces equal and so no movement, anything less and the bubble is pushed out the bottom.
...to read the article linked to in the previous post which provides a more detailed and interesting explantion.