Is CO2 a must

When growing indoors...in a closet or a grow box, is it a must to have co2 circulating while the plant grows or can it go without it?

extra co2 it can go without need some though..no need for the bottles

you will have enough co2 in the air just as it is, some way of bringing new air in will make sure there is always a supply of co2 for the plants. for most growers co2 is not used. maybe just the bottles around flowering time for a small boost but really not needed.

Air inherently contains CO2 so, yes.
You want to constantly circulate air to move the small air molecules off the undersides of the leaves.
Implementing an external source of CO2 is not necessary.
But, it is beneficial and there is plenty substantial evidence to prove.

As long as your supplying fresh air though, you will do fine.

dont get me wrong, a totaly sealed, cool tube like vented grow area with the ability to flood the room with co2 for long periods without venting it out would be the best possable thing you could do involving co2. the more co2, the more bonding agents for the plant, so you will also need to feed more. for a small closet grow or a grow tent it would still increase the yield but if you dont have the money for a system then dont worry about it. just make sure you have a fan bringin new air into the room and one to blow the new air to all the plants (oscelating fan).