I have been through this problem as well. I have gone through
"precision needle valves" and other junk.

The best solution of commercial equipment for planted tanks in the US
is the Eheim regulator with diffuser. I use it in combination with a pH
controller that is almost unnecessary with the stability of this unit.
I use a Dennerle Check valve for safety too. drsfostersmith carries
this unit with a us co2 tank adapter.

I have used dwyer needle valve, super precision needle valve, but the
eheim step down is very stable. I also use one on a calcium reactor
with excellent results