Another conclusion I draw from what I saw, and my thinking about it: a pendant light delivers a very wide range of light to the tank, depending on how high it is above the waterline. So, if the closest you can get the bulb to the waterline is 4 inches, you can cut the intensity in half by just raising it to the square root of 2 times 4 inches or a little more than 5 1/2 inches. For that situation, which is pretty typical, raising it 1.6 inches cuts the intensity in half. I had not realized how sensitive the intensity is to the hanging height.

However, another aspect of this is that a very powerful light, suspended 16 inches above the waterline, will lose only about 3/4 of its intensity by the time the light reaches the bottom of a 16 inch deep tank. Now, think about the problem lighting a 36 inch deep tank imposes! (Makes me wonder if WWII surplus carbon arc search lights would work best!)