You can do this by creating a new PL gate.

1. Go to the PL settings page and choose an unused gate
2. Set the function to AND.
3. Set Input 1 to the PL gate which contains the pH probe and timer function.
4. Set Input 2 to Filter and choose the feed pause number, then save settings.
5. Go to the Switch Channel page, choose the socket for your CO2 reactor, set the function to the PL gate you created in step 4.
6. Save settings.