I agree with redchigh; personally i would not waste the expense of CO2 in so small a tank. And I would worry about fluctuating water parameters more in smaller tanks than in larger where CO2 diffusion would have more benefit. But it all depends upon what you want from your plants, or the tank in general. The majority of plants will grow fine and be healthy (thus performing their function as water filters as well as looking nice) without adding CO2; there is a lot more CO2 produced in a healthy aquarium than many realize. But if you want very fast plant growth for some purpose, like propagating plants to sell, then adding CO2 will help. But remember that adding CO2 means considerably more light is needed, and daily dosing of nutrient fertilizers. This means more expense to set up and operate daily, plus the higher level biologically. The more you add, the greater the risk should something go wrong.