I'm not sure what's going on with my needle valves.
I distribute CO2 from one bottle to 2 tanks and I set valves to specific bubble rate. Later I found that bubble rate dropped and drop is quite significant, 1/3 even 1/2 of initially set.
I even tried to set bubble rate higher and next day it dropped to wanted rate, but one day later it dropped even further
Happens to both bubble counters. Pressure before needle valves is 2 psi.
I checked for any leaks and found none. System is turned off at night, but didn't found any correlation with that.

I have had similar problems with needle valves in the past. What has worked for me was to close the needle valve then open it to full flow (if you are using a diffuser make sure to remove it before turning up the flow), then adjust it back down to whee you want it. In my experience, it seems that something causes a blockage in the needle valve and cranking the CO2 up for a second blows it out.

I have also had luck by increasing the pressure on my regulator when changing from a single needle valve setup to a multiple needle valve setup.

I think the problem is the 2 psi pressure at the needle valve. With such a low pressure, the flow through the valve is dependent on a lot of factors other than the geometry of the flow passage. And, those factors are not stable, so the flow changes. I found I got much better results with a pressure around 20 - 30 psi at the needle valve, even though adjusting the valve is very difficult, with my cheap Milwaukee regulator, compared to at a lower pressure.

May want to check out Rex's Guide to Planted Tanks for CO2 supplies. VERY pleased with his needle valves, check valves.
Angels

Ah, not psi it's bar. Preassure is 2 bars. That's enough preassure I think.



I'm from Europe, doubt Rex stuff would be compatible with my system

I agree. Two bars should be enough pressure. Now it sounds like bad needle valves.

A good valve is a happy thing and a very critical part of any system using CO2 gas.

Seek a good one.

Regards,
Tom Barr