heat stress is most likely the #1 killer,,, and that is usually due to poor ventilation... especially with grow boxes like yours... at least when you are running a grow room you can walk into it and say "wow! this is too hot" and than do something about it. If you do not closely monitor the heat and humidity levels in your box you will not know what it feels like inside so i would also suggest a thermometer and something to measure the humidex