Except for a new in line diffuser the generator is from my old system of in tank ceramic diffuser. The tube is actually 4/6 mm CO2 hose, not ordinary air tube. Here not yeast involved yet nitric acid reaction over sodium bicarbonate. With this 2 fizzy bottle system the pressure could be adjusted and maintain by increasing the right amount of nitric acid to baking soda. Here the pressure is maintained at 1.5 bar (22 psi) and I found out enough to get CO2 equilibrium in by 50 gal tank in about 2- 3 hours depend on bubbles count/ sec with this particular diffuser. I have a timer set to get the CO2 be saturated before the lights kicks in and shut off the CO2 three hours before light goes off.
I am waiting for my stainless steel cylinder now and the generator will be different yet the CO2 maker chemicals will be the same. Nitric acid/sodium bicarbonate works very well.
You are right about bubbles counter as a check valve, albeit any check valve works, I prefer the proper one for CO2 with low opening cracking high pressure resistance.