Quick double check on CO2 levels... toward the end of the photoperiod (after CO2 has been on for several hours - shortly before you leave work), pull a cup of water from the tank. Do a pH test on it, note reading, retest another sample from the same cup the next morning. If you *had* about 30ppm CO2 in the water at the end of the day and this has dropped overnight to the 3ppm that is typical of standing water at atmospheric equilibrium, your morning pH should be up about 1 full unit. If it only goes up 0.5 unit, you only had about 15ppm CO2 before. If it doesn't move, it doesn't mean you have no CO2 variation, it means your tank's buffering capacity if high (high GH and/or KH) and that this test won't work for your tank.
If you're going to have CO2 dosage problems, it's best to deliver too little as too much can kill your fish. Drop checkers need to be very clean as the 4dKH solution used in them has pretty low buffering capacity. Any soap or other residue can result in unwanted buffering capacity within the test chamber thus rendering results inaccurate. When in doubt, clean it out thoroughly, rinse it w/ lots of DI or RO water, rinse it a time or three w/ a little of your 4dKH solution, then fill it and add your pH indicator drops.
You need not repeat this procedure unless your reagent fades (place drop checker low or in a shaded region of the tank to prolong reagent life as it is photosensitive) or gets contaminated (yes, snails may investigate the interior of your dropchecker and yes, they do get overturned now and then).