As squid says, co2 isn't always essential. In your case at 1WPG its not necessary at all, although I dare say the plants would appreciate the extra CO2.
For a 40g I would want a pressurised kit. You'd probably need 3 yeast type kits to get anywhere near enough co2 into the tank, and changing those would be a hassel. There is a link in my sig to how to do pressurised Co2 on the cheap