hells bells guv, when the lights are on the plants breathe in co2 and breathe out air, when the lights are off they breathe in air and breathe out co2, so without a constant air supply in the tent and out again they are getting all confused, it wouldnt have to be much of a fan but you would have to leave it running all the time, that is presuming you are not having any heat problems from your light. so best to work out your tent total area say 4 x 4 x 2 meters thats 16 cubic meters so you need a fan that will empty one third of that area (say 5 cubic meters) a minute every hour so you need a fan that will run at 5 x 60 = 300 so you need a fan that will empty 300 square meters an hour, presuming you are using a carbon filter your exhaust fan would need to be about 500 square meters an hour, so how does that grab you. and that will help sort any heat problems too if the air going in is nice and cool and fresh. if your tent is 1 metre x 1 metre x 2 metres then 150 cubic metre an hour fan.