You can use baking soda (sodium bicarbonate) as a carbon source or compressed CO2 on a "pH-stat" system that administers CO2 based on the pH of the culture (set it at 7-8). Continuous drip systems with compressed CO2 are also possible.
For pH stat, you will need a pH probe, regulator and solenoid valve controlled by a pH controller. (see below for a controller)
http://www.aquaticeco.com/subcategor...CO2-Controller
For small scale cultures like the Aqua Medic reactor, I prefer either sodium bicarbonate or pH stat.
EDIT: I should also say you do not need to use CO2. You will get a higher culture density if you use CO2, but if you do not you can still grow algae well.