In one tank I inject into the return line from the sump. As Lepomis said this isn't so far removed from an out of tank reactor. The total water volume is a little over 10gals and I use 2 2L bottles. (I also have outgassing due to layout.)
In a 10g I inject into a Hydor Pico powerhead, which is noisy, but its actually become more like white noise to me. The pump is on the same timer as the main lights. This tank has 2 1L bottles.
Another 10g is a ~med/ambient sunlit tank, and it used to get CO2
from Hagen ladder, injected into the media compartment of an empty Cascade 100 filter, and then an empty Nano filter. I used 2 1L bottles and just thought of the CO2
as a bonus, so did not take efficiency or regular mix changes seriously.
Currently have 1 2L bottle going, and may need to up it to 2 when I get the 2nd tank going.
I may misunderstand, and I have not tried this, but if your plan is to use one set of DIY CO2
bottles for multiple tanks, I'm not so sure it'll work. I suspect since the pressure/output is not constant and there is no resistence to the line, most of the gas will go to whichever diffusor has the path of least resistance.
Better may be a manifold and separate needle valves, so you can branch your pressurized CO2
to multiple tanks while controlling levels. Rex Grigg's prices seem reasonable and he'll give advice: http://rexgrigg.com/sale.html
Also, with pressurized you know glass diffusors will work.