Join Date: Jun 2008
Location: Orange County, CA
If your plants are healthy and you don't have an algae problem, then I don't think you need to add CO2. It is true that it could help your plants grow more, but if everything is ok that just means you will have to prune them more often.
At 2.7 wpg or more, you will probably need some form of CO2. For a 46 gallon tank, you can connect 2 or 3 2L bottles of yeast mixture depending on how high you want your CO2, and swap one out every week.
For buffering, it depends on what the pH of your tank is normally. My fish need "soft and slightly acidic" water so I used distilled water mixed with a little tap water.
And as for turning your CO2 off at night, with a DIY system that is not advised but can be done with a pressurized system. I use a DIY system so I do not turn it off, but even if I could I would not since I never see my fish at the surface gasping for air before the lights come on.