You'll likely find that the lower Kelvin-rated bulbs will last longer than the 20K's. I've been told by bulb manufacturers that the phosphors that are used to produce the blue color for some reason don't last as long as the ones that produce other colors. Since the 20K's are so heavily geared towards the blue end of the spectrum, I've heard of them lasting as little as six months before needing to be replaced. In addition to that, you mentioned the growth of your corals. Remember that the spectral plot produced by bulbs that are lower in Kelvin temp (I'm thinking of Ushio and XM 10K's in particular) tend to be much more even across the board, where as 20K's often just have one big spike in the lower (blue) end of the spectrum with virtually nothing in the other colors. That is very different from the lighting that corals would naturally receive; they have evolved for something very different. If you really want the vibrant colors that bluer lights often produce, adding extra blue (T5 or VHO maybe) to lower Kelvin-rated bulbs would give you that color without sacrificing the light in the other wavelengths.
Hope that helps,
Clayton
-filling in for Steve while he's on vacation
Hope that helps,
Clayton
-filling in for Steve while he's on vacation
