The pressure you use will vary a lot depending on what and how you work the CO2. Some diffusers need high pressure to do anything at all. Others like a reactor just need enough pressure to let the gas flow into the water stream. For my various tanks, I've used as low as 4 and as high as 15 to feed my reactors. It will be a combo of what pressure, what needle valve, and what method of diffusion is used. Just take some time to play it up slowly while watching the fish is the way I go. I might suggest 15PSI to start and adjust from there.