not sure why someone would tell you not to use the 2nd fan. As the above post stated "they are air tight" so, if they are air tight and you are sucking with the one fan to take the heat off the lights, and the tent is air tight you are taken away all the air from your plants.
Think about it. A well made tent is nearly air tight. Your fan is pulling air out of the tent at a good clip. If the above statement were true then as the fan sucked out all of the air, your tent would crumple up into a ball! Sort of like sticking a vacuum tube into a plastic baggie. Cut a hole in the baggie (open a few passive vents) and the air pressure is equalized. No need for an intake fan. Note that this is true even in situations where the grow room is not very well sealed in which case the CFM rating of the fan will be the determining factor. Only the largest grow rooms need to push air into a room. Balancing negative and positive air flow with two fans can be very tricky, especially if odor is a concern. Create negative pressure with the out flow and you will be good to go.

If you can not return the second fan here is an idea. Use fan 1 to vent the lights in a closed loop whereby the fan intakes fresh air from an external vent, passes through the lights, and exits. This assumes that you have glass in front of your lights. Use the fan 2 to vent the tent either with or without a carbon scrubber as per your requirements. As GToD spelled out, you will need to open vents for passive intake - the rule of thumb is 1.5x the external vent so if your external vent is 6" then your passive intake would be 9" or better. It's up to you. Hope this helps!
bigsby Reviewed by bigsby on . Grow tent Hello everyone,. I am starting my very first ever grow. Here is what I have so for. One Secret Jardin rd300 grow tent. Measure just under 10x10 with a roof space of 6'7. I have two air cooled 1000w lights and two 1000 w Lumatek Ballast. Also one 6 inch can-fan for the lights, and number fans for moving air inside the tent, one 6 inch can fan for push air into the tent,. Last stage buckets will be 5 gallon size. Soil with be foxfarm, nut will be from advance. PH etc at home has been Rating: 5