The less surface agitation the better when you are injecting CO2.
Surface agitation helps vent CO2 into the atmosphere.
My air stones are on timers and only turn on when the lights and CO2 are off.
This is all subject to stocking rates of course, If you need an air stone on during the day that's fine but try to make as little surface movement as possible.
A bubble counter is a necessity IMO, it allows you to accurately measure the amount of CO2 that you are injecting into your tank but if you don't want a diffuser you can always release your CO2 into the intake of your FX5 and let that diffuse the CO2 for you.