Yeah, but there's a HUGE range of color temps available in MH. From as low as 2700K (which is very warm/yellow-red) to as high as 20,000K, which is almost completely blue. These bulbs that are made in the higher Kelvin ranges (>6,500K) are all made for aquarium use, specifically to grow photosynthetic aquatic organisms--corals, clams, corallimorphs, anemones and other Scleractinians. What does that MEAN?
It means that these bulbs were designed to mimic equatorial (tropical) sunlight as it passes through water. Are we growing Mary under water? No, she's terrestrial and what she needs to flower is the first thing that's lost when light passes through water--the red end of the spectrum. No flowering plant can really flower without red spectra, in fact.
So, while some push of UV can help keep internode spacing tight, that can be achieved in other ways and it does not require an MH of 14,000K to get. All metal halide bulbs push UV light, all of them, and all HPS bulbs push zero UV light. So you can use a MH bulb of 4,000K (like the ones I get from Lowe's) and that could actually, in my currently limited opinion (because I haven't yet tried out my 10,000K 175W MH, cabbaged off a friend's reef tank hood) do you just as well as the 14,000K.
That is why I said that it seems awfully blue to me, because those high Kelvin rated bulbs don't usually have as much red spectra available. It is also said that reefers can experience microalgae, diatom and cyanobacterial blooms in presence of too much red light.
HPS don't cause stretching, it's something that happens naturally whether under the Big Metal Halide in the Sky or under artificial light. I found that MH does not prevent stretching, and saw no appreciable difference in trichome coverage between tables where I split MH and HPS. I think those aspects are a bit more difficult to quantify, and my main goal is to grow flowers.
Did you know that you can give the plants UV by using fluoros and CFLs? Yep, tis true.
