~ To answer the first question I find the best thing to do is go by the size of your planted tank, I personally have a 15g, rather small planted tank but has a lot of plants too, I use 1 liter bottles with 1 cup of sugar to 1/4 teaspoon of yeast, you can scale this to your tank size as reference. I keep 2-3 bottles of sugar water ready to mix yeast in anytime, that way I get a week and a half to two weeks before I switch the yeast and sugar mix out for a new one, also you only have to pour out the bad water and keep the yeast culture in the bottom, then add more warm sugar water and mix it up again. This can be done a few times, I do it twice and then replace the yeast along with the water and sugar.
~ To answer the second question yes you shut off your water pump during low light or night time hours because your plants actually release their CO2 during those times, if you don't you can build up carbonic acid in your aquarium lowering the pH, if you run you system non stop, also you inhibit the hemoglobin in the fish's blood from absorbing the oxygen in the water if to much CO2 is present in the water. You can watch the CO2 level by watching kH or pH levels. Low pH means High CO2 levels obviously. What i do because I only get a bubble every 2-3 minutes I let it build in the dissolving chamber until the pump comes on in the morning with the lights, to do this you don't need much CO2 generation but a very efficient dissolving chamber that is sealed completely at the top to hold the CO2 until used and be large enough not to blow out the CO2 during start up, hope this helps.