You can always spread the wealth around to others; get them involved with fish. If I remember correctly, plants absorb CO2 by day through photosythesis and revers the process at night. CO2 imbalance due to too many plants never came up as an issue. With regard to the nitrogen cycle, you can purchase a test kit; it wouldn't hurt. I wouldn't mess around with chemicals to speed up the process. It isn't necessary. Just set up ypur aquarium with the usual hardlines: gravel, thermometer, heater, filter, pump, light... Add the water and let the tank filter for a couple days. Add a plant. Add a fish - cheap one that can be sacrificed - like a FEEDER guppy. Now we can talk about the nitrogen cycle. The guppy will produce waste products - namely ammonia. You may notice that your tank clouds up. Don't worry. Nitrosomonas bacteria beaks down harmful ammonia to also harmful nitrite. Then, as the ammomia concentrations go down, the nitrite level goes up. Nitrobacter bacteria breaks down the nitrites into much safer nitrate. When all this occurs it is truly safe to introduce more fish. This process takes a few weeks and it also takes place on the filter media, for the most part. A dirty sponge filter or whatever is a GOOD thing. A brand new filter looks nice but it is not "seasoned." That is, it hasn't gone through the nitrogen cycle. A dirty and full filter; that is, clogged, is a filter that isn't working. It needs to be squeeezed out in the sink. I learned this the hard way. I also learned that using "aged water" on a new filter is anice temporary fix - but a really bad idea. It has no "good" bacteria, meaning it hasn't gone through the cycle. Don't try to rush the process. Be patient. Go slowly. Add fish slowly. When I was a kid I had a 5.5 gallon tank with a cheap incandescent light on top. In it I placed a few broken, leftover pieces of water sprite. they floated. After several weeks those "pieces" grew and grew and grew - above the water line. An airstone is nice to have I don't see any reason to get a smaller or larger one. Just get a filter going, even a small, cheap sponge filter and let the bacteria deal with the toxins produced by animal waste products. Have fun!