CO2 diffuser pump size???

I am thinking of adding a small sub. pump and a spray bar
to my tank to be used as a CO2 diffuser, and to eliminate
some dead spots in the water circulation. I use a Whisper
outside filter now and my tank is 24 " deep so the flow at
the bottom is nill. Anyway, I use DIY yeast CO2 and I think
I waste much more than I use. Would this improve the absorb-
sion? Also what flow rate should I try to get? The tank is
40 gallons, The water is about to be supplied by RO/DI so
I can adjust to any DH/KH I want.. Any Ideas???