You will need a drop checker.
When the fluid in it turns from blue to a green color, you will have sufficient CO2
I have a Milwaukee unit and diffuse using an Up-Aqua diffuser on the intake side of my Rena XP2 canister. I also have a Milwaukee SMS 122 Ph controller.
The controller will turn the solenoid off when a preset Ph level is set. The Ph level sort of indicates the amount of CO2
that is dissolved in the water.
You can also use the solenoid with a timer to turn on the CO2
injection when your lights are on and off at nighttime. Instead of the Milwaukee solenoid, I use a Burkert 6011 solenoid which a lot of aquarists' desire over other brands. I am not happy with the Milwaukee supplied needle valve, I recently ordered a Fabco NV 55 needle valve from Green Leaf aquariums. ALWAYS use a check valve behind your expensive regulator. Water will siphon back and do major damage to it.
You can look for the foreign made Up-Aqua diffuser on Ebay for under $20 shipped:
Use caution when starting your pressurized set up at first, I gassed some angels. Luckily for me, I saw them floating upside down and sideways, threw an airstone in and revived all of them.
On my smaller tanks, I still use DIY CO2
(yeast, sugar baking soda and water). You can find numerous recipes on plantedtank.net or aquaticplantcentral.com.
Some folks say that shrimp are sensitive to CO2
. Mine are NOT.
PS: Didn't you find a regulator on Craigs list? I don't have experience with that one, but it should work.