Sam,
I got a nutrafin kit, then made my own DIY one a few days ago. They both work equally well, but to be honest the DIY is no way as nice and tidy and doesnt have the diffuser! For £20 for the nutrafin u cant go wrong, imo well worth the money, especially if your gonna fork out for the diffuser anyway! Only think is, remember to modifiy it to add a waste tank/cylinder just incase!!! (suppose thinking about it, that requires DIY anyway!! )
Gwikim - i understand that if you push the CO2 through a stone it will make smallers bubbles therefore allowing more surface area for diffusion, but with CO2 being relatively hard to dissolve is it not more about how long the gas in the water for rather than the surface area of the bubbles! Even tho the bubbles are smaller are they not still just gonna shoot to the top.
I think an upturned jar sounds a better option or like you said (and the way i do my DIY system) - through the filter inlet
Very interesting!
Good luck,