I thought I would share my experiences with using this method to aerate wort and yeast starters.
The method is very simple. You use an aquarium air pump, vinyl tubing, a stainless steel air stone, and a HEPA in-line air filter. You hook it all up, and drop your (sanitized) vinyl tubing with attached air stone into the wort. Plug in the pump (making sure to use a drip loop), and let it go for a half hour or so before pitching your yeast.
I wanted to try this method, because my previous method of siphoning the wort through a screen and shaking the primary fermentor didn't seem particularly effective to me. Not to mention shaking 5 gallons of wort is a PITA.
The first issue was finding the filter and air stone. Air stones used in aquariums are made of some kind of porous stone-like material that you definitely wouldn't want in your wort, if only because it would be nearly impossible to sanitize (that and little pieces are always crumbling off). I finally found a stainless steel air stone here, and an inline filter here. Once you add in shipping, this is quickly becoming a rather expensive undertaking.
More problems arose once I had the equipment and tried to use it. First, the air stone and filter have 0.25 inch fittings, and I'd gotten quarter-inch vinyl tubing to fit them. I'd forgotten that air line tubing used in aquariums is much smaller (I believe an eighth of an inch). So I had no way to attach the filter to the air pump. Luckily, in a stroke of sheer brilliance, my fiancee realized (as I was cursing my failure to notice the different sizes before brew day) that the orifice of the filter fit over the outlet of the air pump perfectly.
That problem solved, I hooked everything up and plugged in the pump. Nothing happened. Or rather, nothing seemed to happen. Upon close inspection, I was able to tell some air was coming out of the air stone, but not much.
I immediately suspected the filter. Perhaps the air pump wasn't powerful enough to push the air through the filter. I'd only gotten an air pump designed for a 10 gallon aquarium. Maybe I should have gotten a bigger one. Figuring that a little air was better than none, I finished my project (a yeast starter).
After the starter was done, I examined the system and found it wasn't the filter gumming the works, it was the air stone! I'm a trumpet player, so I can blow pretty hard, and I couldn't blow air through that stainless steel stone to save my life!
As an experiment, I tried hooking everything up, minus the airstone, and it worked beautifully. I'm now using the set up to aerate another yeast starter. I just have the vinyl tube in the jar, with no air stone. It works like a charm.
In aquariums, the air stone turns the large bubbles that would normally come out of the vinyl tube into a host of smaller bubbles. This, I imagine, is partly to look cool, partly to not freak the fish out, and partly to improve air exchange with the water. As a brewer, I don't care how it looks, and I don't have to worry about scaring fish (if there are fish in my beer, I have much more serious problems to worry about!).
Now, I imagine a host of smaller bubbles WOULD provide better air exchange than the 0.25 inch bubbles. However, the stainless steel air stone was a total fail, and a bunch of big bubbles is definitely better than few to no small bubbles. Also, without the air stone, the bubbles are coming out forcefully enough that they're effectively stirring the solution.
Maybe a larger air pump would be effective with the air stone. Perhaps one made for a 100+ gallon aquarium. Has anyone tried this? What were your results?
Barring that possibility, I think using the air pump and filter sans air stone is an effective way to aerate your wort, especially if you add it to the straining and shaking method. In a small jar, like for a yeast starter, it is also a decent way to keep the yeast in suspension (along with occasionally stirring/shaking the jar) if you don't want to shell out the money for a stir plate, or don't have the ability to make one.
Not buying the stainless steel air stone greatly reduces the cost of this venture. Also, you can make your own filter using a tube and wet cotton balls (as described in Palmer's How to Brew), which would save even more money.
All in all, I'm happy with this method. Some day I may try a bigger pump, but for now I'll just omit the air stone. If I try a bigger pump, I'll report my findings.
If anyone has experience with this method, please let me know how it worked out for you, and how you feel it compares to other methods.