I keep 1 24 watt ho t5 taped on top of the humidity dome on my cloner/seedling starter and after that you can keep it as close as heat management allows...I personally wouldn't go much closer than 3 inches regardless of what people say about how you can basically stick these on top of your plants...look at and learn to read your plants and it will tell you if there's too much heat/light. I have a 200 watt 2 ft 8 bulb ho t5 in a 3x3x5 tent and I have it fixed to about 8 inches from the top of the tent...I never move it and never have any issues with stretching/long internodal spacing