i am not a fan of the glass diffuser, i feel as if there are better methods of diffusing. such as the reactor, power head, or running co2 through a canister filter.
but to answer your questions: bubbles/second or minute. larger the tank, the larger the diffuser
powerhead? i never heard of this method. can you put into more detail?
SURE!
really short version: have the co2 output right below or in your powerhead intake, impeller chops up the bubbles into tiny bubbles and shoots them out.
slightly longer version: this method might cause a loud noise to come from your powerhead due to the impeller vibrating from hitting the air bubble. to fix this issue you may want to place an airstone at the end of your co2 tubing in order to send up smaller bubbles into the powerhead and less noise will be made.
a little longer version: this might cause you to miss some co2 bubbles that dont get sucked into the powerhead. find a way to shove the airstone intto the intake of your powerhead (if it doesnt fit you can probably use a 1/2" bulkhead strainer as your new strainer, drill a small hole into the bottom to allow the co2 tubing to be placed into it
the 'little longer version' is exactly what i do....i think i have a picture someplace....ill see if i can find it.