Very warm...Kind of humid...

I'm a very well educated and experienced in the growth and growing marijuana. I am growing again and limited with options this time. I have a chest that is 2X5X2 that I am starting the plants in. I have 4 Master Kush strain plants. The box has 3 flourescent lights...I know not the best lighting source but they work. It still gets to be very warm. I have 4 ventilation holes and a fan blowing air out with the lights going 18/6. 1 plant is growing well but the leaves have been burning on the lower leaves while it still grows and has a great color and smell. I have another place to transplant them to for the rest of their growth, but I don't want to have the leaves burning. I have checked every possible problem that it could be but none seem to explain the problem. Anyone have tips on how to fix this?
Thanks




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What are you growing in? Is it your standard soil mix?

Add an intake fan to increase ventilation and control temps?

Air conditioner