also i forgot to mention that photocnithsis only ocurs when light is presnt there fore when in darkness the plant dosent use photocinthis so the plant dosent need high levels of co2 to grow alough it does still need to breath,if the grow room is ventalted good enough, i would not advise you to use a fan in darkness.considering it is not much needed and the plant is resting in darkness wich helps produce bud growth.co2 levels are one of the main aspects for plants deveoplment,the more co2 the helthier bud and higher yields.

p.s this is a breif statement, i have not fully gone into detail,also sorry about the spelling,
good growing