I have to say, and I wish more people were aware of this, aquarium pumps just do not aerate the wort as well as hand shaking. Somewhere in the archives of basic brewing radio, there is an interview of a guy who did some side by side tests of shaking versus aquarium pumps, and the aquarium pump dissolved oxygen levels were not nearly as high as shaking for 5 minutes, even after a half hour!
I have made some of the shittiest beers I've ever made using an aquarium pump, including a really fuselly barleywine. I threw the pump out, kept the stone it came with, and went back to aerating by shaking until I got a basic oxygen system. No problems with bad fermentations when I went back to shaking, but for about $30 you can get a basic oxygen setup that will improve fermentation a bit, especially for bigger beers.
