oh and another thing, dont forget the bubble counter ( which is kinda like another bottle filled with water to allow you to count the bubbles ) ive read somewhere that yeast might form on the airstone if you dont do this, i think the water filters out the spores or something im not sure didn't really try without the bubble counter.
Ive also read that article and since i had a HOB filter i scoured the net trying to look for a solution, i found a few articles on the other forums that actually say that HOB filters are quite good dissipators of C02 AS LONG AS you fill up your tank to the brim of the downfall part of your HOB so that the water doesn't splash too much.
And yes i think that its a good idea to keep the bell far from surface agitation but near water current ( well this is only my opinion but i think its better if new water always goes under the bell so as to provide more water to dissipate c02 into )
BTW what filter do you use ?
and as always the best way to find out if your c02 diffusion method is working is by testing the water, who knows maybe with just the upturned bell near the current your getting 15ppm of c02 which is great !
sombody correct me if im wrong