Does small grow tents need added CO2?

No, unless the tent is sealed its a wasted product and if your indoors the CO2 levels are already elevated vs outdoors so no need to add in more to your living environment. The cheapest option that works well is legit CO2 canisters with a controller on them, all the hippy products using yeast or fungi just don't cut it, yes they increase co2 by maybe 2-30ppm. but you need 100-1000PPM. Good Luck!