I finally bought a descent test kit (Aquarium Pharmaceuticals Freshwater
Master Test Kit) and tested the water after the lights had been on for about
11 hours with the following results.

Try testing your water first thing in the morning, just before the lights go
on. Then test it in the middle of the photoperiod. If you start out with a
lot of CO2 in the morning, and it decreases throughout the day, until lights
out, then the test kit is probably accurate, the Hagen thing is working, and
your plants are simply using up the CO2.

What can I do to raise the CO2?

Bigger batch of yeast/sugar solution, or two bottles of it, running staggered.


Leigh

http://www.fortunecity.com/lavender/halloween/881/