I was told by a lasy who works in a commercial greenhouse, where they aerate their water constantly, that my 40 gallon res should optimally use two 6" stones......one at each end. Anything more and you can lockout or root damage your plants. So....if 40 is 12".........that means 3" per 10 gallons rougly........just what I was told. I ask alot of questions and research alot. The one thing I am SURE of is that the smaller stones that produce more bubbles are the better route to go. This has been stated in all my research. I know in aquariums, as well, that more small bubbles is better than fewer large ones...so makes sense in plants too. Hope I offered help.