You need another fan to exchange the air. Along with passive air intakes or another fan to suck fresh air in. So possibly 2 fans, an exhaust and an intake. your choice. The fan on the hoods is just sucking your heated air off your lights im sure you understand that. When running co2, time your new exhaust fan to shut off while the co2 is on while leaving the fan running on your hoods so the heat doesnt build up. With your air cooled reflectors it shouldnt waste your co2. If you dont feel like your hoods are sealed enough go out and smoke a bowl and see where it is sucking the smoke in through the hoods. If you find the smoke getting sucked through you hoods seal them off with something so your co2 isnt lost. fresh air exchange is important. If you dont have any you are just stuck with stagnant air around your plants and thats not a good thing. I use passive air intakes with a piece of duct work with a flap that opens when the exhaust fan is on, when the exhaust fan is off the flap for the most part shuts allowing the co2 to build up for the amount of time it takes to reach your desired co2. think of fresh air exchange like this:
Put a bag on you head and breath for a short spell. You are gonna find it hard to get a good breath of oxygen. Your plants could suffer from the same thing.
