My tank height only 13 inch ... and I am using 250 DE ..... One bad thing I notice is the water get hot pretty fast ... even though it is 20K lighting .... becasue the general perception is that a 20k bulb have less par so we tend to lower the lighting to the surface of the water and the heat emit is the same as a 10K bulb (only slight difference ). My chiller switches on and off like nobody's business.
Anway, I am testing it out what is the difference ... may switch back to 150 W DE one day.