I just setup CO2 on my new tank. Do you usually run this all day or only when light is on? my tap water is 7.4, I just ran for few hrs today and now my tank is 6.8
On 30 min before lights on. Off 30 min before lights off.
+1. I do this also.
If I keep it all day, will it have a stable PH and how low can of ph can it get?
Injection of CO2 can only lower pH to a certain value (I forget the precise value at this time).
If I keep only when light is on, how much swing in ph should I expect? and will this swing affect small tetras (Neon, Rummynose) and cherry shrimps?
In general, pH swings that are caused by CO2 will not be harmful to livestock.
Rather than watching your pH, I would recommend that you use a drop checker with a 4 dkH reference solution so that you can first ballpark the CO2 levels you have, and then monitor your livestock carefully from there to see if you can increase (or you may have to decrease) your CO2 levels.