i fixed the regulator, the needle had things on it, now it's dead stable, i think that i might save so co2 using inline ?Since you already have a working CO2 system using an in-tank diffuser, there is no harm waiting 24hours before adding the in-line diffuser so that you can have a 'stable' bubble size and start tuning CO2 straightaway.
Also, as your tank is only 51 litres, I don't think that there is much of a difference between in-line and in-tank diffusers for a 51 litre tank, so there's no hurry. My bigger concern is your 7-year old regulator that is giving you problems - i hope you have got a new one in addition to your new diffuser?
update, it worth waiting, the bubbles are so tiny, it's a mist, incredible, way better than in tank for me, co2 are everywhere, even my myriophyllum tuberculatum is growing =)
tuberculatum is high tech plant, it need lots of co2 and fertilizer, no dead leaves for the moment, will see, the pump on the right gonna be removed, i changed the scape a bit, as you can see tuberculatum seems finethats great - do share some photos of your tank and the Tuberculatum. Is the Tuberculatum easy to maintain? Does it shed a lot of dead leaves?