Subject: What is the effect of a plant on CO2 level in a closed system?

I am working on a science fair project to see what absorbs the most carbon 
dioxide in a closed system.  I am testing plants, salt water and grass.  My 
system is a 20 gallon aquarium that is sealed with a sheet of clear plastic on 
the top and clay around the edges.  I put a carbon dioxide meter in the 
aquarium and sealed it shut for 24 hours. As expected, the CO2 level stayed 
relatively constant at around 6-700 ppm.  When I put a big potted plant into 
the aquarium and sealed it, I expected the CO2 level to decrease because plants 
use it to grow.  I was surprised to find that the CO2 level increased at a 
steady rate (about 200 ppm/hour) for the duration of the experiment, about 24 
hours.  Can you explain why that would happen?  Could decomposing matter in 
potting soil cause this effect?

Re: What is the effect of a plant on CO2 level in a closed system?