It's best to use two smaller diffusers at each end of the tank and have their own matching Canister filter out flow.

As far as reaching your pH set pioint on the pH controller, you need more CO2 added, the controller will shut it off when the pH gets to the set point.

So you need to turn the needel valve bubble rate faster for now, and then let the pH controller do it's job, if you limit the bubble rate, the diffuser can never match the set point pH on the controller.

You always set the bubble rate higher(some do is slightly, some fairly faster) than you need, so the pH controller can throttle the CO2 within a target range.

Regards,
Tom Barr