I put my glass diffuser near the canister intake - run mine at 130 - 140 bpm and have a ppm of 24 - 28.
I was also very nervous when I introduced pressurized Co2 to the tank. I started out slower and worked up. My drop checker is the standard - I watch it pretty closely.
I have "never" had a problem (knock on wood) with o2, fish gasping, etc. I set a timer for my Co2 to come on 30 minutes before the lights and go off with the lights. My photoperiod is 10 hours a day - 5 on, 2 off, and 5 on - 7 days a week.