Brookster123
Aquarium Advice Addict
Who runs them when??
If you have CO2 you want to run them at night to keep the surface agitation moving to remove excess CO2. If you don't have CO2 then you can run them 24/7.
That's fine but honestly I would just let it run all the time if you don't have CO2.
If you have CO2 and an HOB filter you may already be losing a lot of CO2 if it's cascading into the tank or causing too much surface agitation. Using CO2 and an HOB can be tricky. I would run the PH at night for sure.