yeast co2

Question
I have been loosing fish, and wonder if it is because of excessive co2.  I have a 2 litre bottle with 2 cups of sugar and half a teaspoon of yeast. I have a 46 gal tank, all values are good, but the ph is very low due to the co2.  I did not know until I read your post that I should not run the co2 at night, and I seem to loose them overnight.  Should I just take the airstone out at night when I turn the lights off?  My bleeding hearts are doing fine, but I lost my pencil tetra and almost all of my neons.  I had a six inch moonlight gourami and a three inch cherry barb that had no problems, but I gave them away so I could start over with some smaller fish that would have been a great snack otherwise.  I do not have an airhose running at present; I was under the impression that it was harmful to my plants.  I have a whisper hang on the back filter.  Does that supply enough oxygen?  I have not had fish for probably 4 years, and though I knew what I was doing then (so I thought) but have forgotten too much!  The two large fish came with the tank, the tank came with an inch of bacteria filled water and a dirty filter, both of which I used to cycle the tank quickly.  I show no nitrites or ammonia, so I am quite sure that the tank cycling is not the problem.

Answer
Ok hey nancy, you seemed to be jumping all over the place here lol. Ok I'll try to keep this simplified as i feel myself trying to tell you all the bad things.. but i'll try to explain all the things you have to change.

1) Yes co2 should never be ran at night, what's the point in running it at night as plants undergo photosynthesis during the day where they use the co2, but at night they arnt using it and your tank is getting to dangerous levels of co2 and causing your ph to fall dramatically probably causing death to your fish. The co2 should only be running aslong as the light is on (10-12 hours a day to minimize algae)  Also what type of plants are you trying to grow, sufficient lighting is required before the addition of co2 over 2.0 wpg (watts per gallon) is when we start introducing co2 for planted tanks

2)figuring out how much co2 is enough is a hard thing to calculate... but if you get a ph tester and kh tester you can figure out how much co2 is enough. When we do planted tanks we want about stable 30ppm of co2.
Here is the table:
http://www.aquariophilie-pratique.net/images/graphics/co2_table.png

You dont want a airhose running when you are injecting co2 because your just losing that co2 so there's no point, enough plants create enough oxygen without the addition of co2.. your just crossing each other out.

How long has your tank been setup for, if it's over 2 months then it should be fully cycled if you did it correctly.

I got a forum at www.fishaquarium.org i'll be able to help you out alot there aswell :)