CO2 concentration in 120g

I've been messing with the idea of a DIY CO2 reactor for a while. I 
finnaly did it. Not all that tough, not sure why I didn't do it 
earlier! Anyway, I've got the 2 liter bottle hooked up to an airstone 
at the base of the tank, it is putting out a continuous stream of 
bubbles. Not enough to disturb the surface a whole lot, but 
noticeable none the less. I don't have a CO2 test kit, so i'm not 
sure about measuring the CO2, but the pH hasn't't dropped much. 
Basically my question is is this enough or too much CO2? the tank is 
a 120 gallon and the plants are not exactly overrunning the tank. 
Quite a few tetras, and angelfish, but no plant eaters. Not too much 
algae either. I've got 5 40w tubes of various spectra, all replaced a 
few months ago. I know I need more light, but can't fit it on the 
tank, also I'm in college, and can't afford all the expenses. 
Anyways, any suggestions?????

rpc7036 at owl_forestry.uga.edu
or rcarter at music_cc.uga.edu

Thanks a lot

Rich Carter, University of Georgia
Warnell School of Forest Resources
Fisheries Department