I just got my air pump and it came with a styrofoam ring type thingy so you can adjust the depth of the airstone. Instructions said to put it at half the depth of the pond, but i was wondering if that was just for winter time. Here is a link to the one i got.
http://www.azponds.com/air_pumps.htm#Laguna
http://www.azponds.com/air_pumps.htm#Laguna