Yes, the valve that stops the water from coming back. On my setup I have the regulator with two gauges, one that shows the pressure in the tank and the other that shows the pressure going out. When I first set my reactor up I purchased a one way valve that ended up not being able to stand the back pressure in the line and got some water in it which caused the valve to become sticky. In order for me to get the bubbles to come out right I kept having to open more and more the "needle valve" on the regulator to get the bubbles to finally start flowing. This caused a couple of problems, once the one way valve allowed the gas to flow through the higher pressure made for more bubbles and eventually that higher pressure would then go down. Once the solenoid closed due to pH being reach the valve closed and got stuck again. I ended up changing out the one way valve and making sure that it was located above the reactor (to hopefully reduce the chance of back flow if it ever went bad) everything has been working perfectly since. Hope you get yours working for you.