You don't need an airstone, well, not all the time at least. Consider the following and it will make more sense (hopefully).
Often times it seems counterintuitive: I mean, you are, after all, injecting CO2 into your tank, and it would seem that fish would need supplemental O2 to survive, which they do need, no doubt, but are already getting as a product of photosynthetic reactions. I'm sure you're familiar with the chemical equation for photosynthesis:
CO2 + 6H2O + Light Energy = C6H12O6 + 6O2
Glucose and oxygen are both products of photosynthesis. The glucose offers "food" for plants to carry out other processes, the oxygen allows your fish to stay healthy and happy. There is an exception: nighttime.
At night when there is no light available, plants don't photosynthesize, they respire (dark reaction). The equation is as follows:
C6H12O6+6O2 ----------> 6CO2+6H2O+36ATP
You can see that your plants are actually using the O2 at night and producing CO2, water and energy (adenosine triphosphate - ATP) (essentially the plants are using glucose to breakdown molecular bonds and release energy). So, since no oxygen is being produced during this period, it may be wise to supplement with an airstone. The easiest way to do this is to set your airstone on a timer so that it comes on around the same time as your lights go off, and turns off around the same time your lights come on; this will eliminate both unnecessary oxygenation and allow for outgassing of CO2 at night when the plants aren't producing O2 for your fish. This isn't necessary, but many people do it, especially if they aren't using a pH controller to monitor CO2 levels. The necessity for O2 supplementation should be dictated, for the most part, by your tank's bio-load, considering both plants and fish.