I did look at that and while I do appreciated the help, I do not understand chart chart at all.
I simply would liked to hang two PAR38 6500K bulbs from pendants above my rimless 40gal tank 37x15x18 and I just need to know how high.
I'm sure from an enters standpoint all this is great info. But, I think a lot of it is lost on most of the readers.
Do you know how much PAR you need or want? Or, at least do you what light demand plants you want? The first chart in this sticky
shows you the PAR value and their corresponding labels of "High Light", "Medium Light" and "Low Light". Once you define that(the type of plants you want to keep), then someone can chime how much PAR you will need with or without CO2. Before that is defined, I cannot assume what you want.
I can help you understand my chart in post #12. The light is set above the center of the circle, the first value that reads 245 in black
means PAR value when the light is 18" from the bottom of the tank(i.e your substrate surface in your case). The value 89 in orange
is the PAR value when the light is set at 30" above the center. When you move along the arrow where it says 9", that the distance from the center of the tank where the light is hang. So 9" from the center of your tank horizontally will give you PAR of 49
when the light is set at 30" above the center of the tank.
In conclusion, if you set 2 PAR 38 15x1W LED with 60 degree lens at height of 30" from the surface of your substrate it is still consider high light range of planted tank and you should use CO2.