well i aint a expert, but the first thought that came to mind would be to use a surface or near surface circulator (like what shorty and bruce have shown before) in conjunction w/ the bottom diffuser. the greatest budget of oxygen influx occurs at the air/water surface (not at a bubble/water surface). by chopping the surface and circulating horizontally, then using the bottom diffuser to circulate (the oxygenated water) vertically, you cover all the bases.
my 2 c.
