Stage II modification:
Was looking for ways to improve the diffusion rate of the system without upping the bubble count so decided to give the ADA "ball glass" diffuser a try.
This piece is different from other ADA diffusers in that the bubbles are not emitted directionally but rather radiate uniformly from a point source. For this purpose it fits perfectly: its a nudge over 4" long (long enough to stick out of the top of the filter) and the ball is maybe 3/8 in diameter (small enough to fit under the impeller cover).
Essentially, we're just replacing the chopstick from the prior post with the ball glass. However, the diameter of the glass stem on the Ball Glass diffuser is larger than the hole we were using before. So it needs to be drilled / reamed out. Since I was getting out the drill anyway, I decided to relocate the hole to increase the water contact time. Here you can see the hole I drilled near the lower left edge of the impeller cover. Drill a small hole then test fit the glass stem and drill again (I actually drilled the hole a tad too large). If you don't have a drill I'm pretty sure you could ream this out with an Exacto knife as the plastic is very soft.
This is what the impeller cover looks like with the ball glass inserted:
It just clears the vertical space:
Snap it back in and fire up the HOB.