I suspected this, thanks Jack.
I might have to undo my modification and test it with some salt water (at a reasonable salinity) just to see how small the bubbles are... I've already got it putting out pretty fine bubbles in freshwater with some time and a couple feet of 4 lb fishing line...
My modified impeller... I suppose this would be called a 'web wheel'...
...works pretty well for making fine (er) bubbles in freshwater. I'll mix some saltwater and see what happens. I have some confidence in this pump's ability as a component of a recirculating skimmer now that I've fiddled with it some.