What is the temp in the tent? I bet it is too high... You should be able to get your light 2 feet from the plants with proper cooling. Increase air circulation or run A/C and supplement co2, otherwise get an air or water cooled light. My buddy runs a water cooled thousand watt fixture in a tent your size and can get his plants about a foot from the fixture, only drawback is he has to run a pump and a 50 gallon reservoir outside the tent in order to keep the light cool.