Progress tonight - There was already an AC70 in the tank for flow/pushing waste towards the filter intakes. The port on the top to hook up the airline is larger than normal airline tubing, and the only section of tubing that I had that mostly fit didn't seem to seal well enough against the inlet to only suck in air instead of water. To resolve for the time being, I moved the suction inlet for the power head above the bubble stream for the sponge filter. This effectively takes large bubbles and makes them lots of smaller bubbles.
I also added an AC50 power head with the air attachment. This worked relatively well, but I need to keep the top of the power head at the water level or the suction isn't enough to draw air down the airline.
The net effect is a bunch of small air bubbles at one side of the tank. Effective but not all that aesthetically pleasing. I wonder if a bubble wand hooked up to a strong air pump would have similar effect but "look" better?