You might try growing some house plants. I know people with green houses sometimes add co2 to achieve faster growth rates. It would be interesting to see how many plants and which types use the most co2. I would love to set up an experimental hydroponic setup on a reverse photoperiod to see what the effect would be on my tank's ph. Just think you might be able to grow fresh tomatoes and raise your tank ph at the same time.
http://www.marvinsorganicgardens.com/breathelife.php
I found a list of plants from a NASA study that determined these where the best at removing indoor pollutants in tightly sealed buildings.
1. Philodendron scandens `oxycardium', heartleaf philodendron
2. Philodendron domesticum, elephant ear philodendron
3. Dracaena fragrans `Massangeana', cornstalk dracaena
4. Hedera helix, English ivy
5. Chlorophytum comosum, spider plant
6. Dracaena deremensis `Janet Craig', Janet Craig dracaena
7. Dracaena deremensis `Warneckii', Warneck dracaena
8. Ficus benjamina, weeping fig
9. Epipiremnum aureum, golden pothos
10. Spathiphyllum `Mauna Loa', peace lily
11. Philodendron selloum, selloum philodendron
12. Aglaonema modestum, Chinese evergreen
13. Chamaedorea sefritzii, bamboo or reed palm
14. Sansevieria trifasciata, snake plant
15. Dracaena marginata , red-edged dracaena