You only have one 600w light and when it's on it's making your temps rise 18 degrees?? Wow that is a huge problem, a 1000 watter will add 10 degrees on average to your grow room so your growing environment must be quite hot. As someone above said, if you don't have any air cooled hoods, get some asap. Also if possible, place the ballast outside of the grow room as this also generates a massive amount of heat. On top of the hood you'll also need a powerful enough inline fan as well as ducting, you need to have an inline fan to suck out all of the hot air generated from the light, this fan needs to suck the air faster than it's being produced, this way it never has a chance to raise your temps. With proper ventilation you should be able to get your temps to 70 or so, however if your ambient temps outdoors are always 80 degrees or something then even with the best ventilation your room will still be as hot as it is outdoors. If this is the case you'll probably have to spring for an air conditioner as well as the inline fan and hood. Sorry to have to be a buzzkill, but if you want this solved properly you're gonna have to spend some more cash.
