Re: Solenoid valve placement?
There are 2-4 reasons why not to do this.
The needle valve can leak and be jarred by the pressure changes.
You cannot control other tanks if you use more than one needle valve without adding individual solenoids as well.
One solenoid right after the regulator will control all the subsequent needel valves that you may have, not an issue if you only have one CO2 enriched planted tank.
The other reason is that unless the system is hard plumbed with copper/brass etc piping that will not leak at all, the CO2 tubing used to connect the needle valve to the solenoid will also leak out a lot more CO2 than normal since it's always under pressure.
I hard pipe my solenoids to the regulator, then add needle valves directly from this. This reduces the pressure and the jarring on the needle valves and CO2 lines.
I've done what you are thinking about, but mainly with yeast DIY set ups and I use the open function to vent off and stop CO2, rather than add it with the DIY set up.
Regards,
Tom Barr