Spectrum is only one aspect of lighting that influences coral growth, and lighting is only one aspect of all the parameters that affect coral growth. Sunlight has a kevlin temp of about 5000-6000K at high noon, only at high noon, the spectrum of daylight changes throughout the day, during dawn hours sunlight can be around 10-12K, then it quickly drops to 4-5K, and slowly rises to 55K up until noon, then it slowly drops back down to 4-5K and then at dusk back up to 10-12K. As mentioned water depth also has an impact on the spectrum of sunlight, not to mention its intensity, water will filter out most of the reds and yellows as the depth increases, and SOME corals that come from deep waters where the spectrum of the light is more blue wil appreciate a bulb with a higher kelvin temp, but some shallow water corals that recieve light that is much more white or even yellow in color may not. This is the theory, coral growth depends on energy intake through prey capture, direct nutrient abosrption, and/or phosynthesis. When talking photosynthesis there are more things to consider than the quality of the light, ie spectrum, you must also consider intensity. A higher kelvin bulb may produce a light more concentrated in the blue end of the spectrum, but may lack in intesity.
It was stated that the higher the kelvin the better photosynthisis growth for such corals.
What such corals? Those from deeper waters may do better under bulbs with a higher kelvin temp, those from shallower waters may gain more coloration, but their growth rate may actually slow. 55K is a good bulb when used in combination with actinics, although I assume you are running the Venture 55Ks, which lack in intensity compard to other bulbs. Keep in mind its possible to keep corals under nothing but 55K bulbs, they don not lack the blues and whites seen in higher K temps, there are just less of them. As Iron mentioned, kelvin rating is really only a general guide line when buying bulbs for MH, I used the Venture 55Ks over my 75 for over a year with great results in softies, sps, lps, and clams, and I really like the color when combined with actinics. I eventually moved all my softies to a 55 gallon with dual 175 ushio 10Ks and isolated the sps and clams to the 75 and upgraded the lighting to 400 watt MH with iwasaki 65K bulbs, the overall color of the tank did not change much, I actually prefer the color of the 55Ks, but the increase of intensity caused the growth rate to go through the roof, but some of the corals actually lost coloration, or "browned out", not a bad trade off though a little color for more growth than I know what to do with
Lately Ive been trying to increase the coloration in my sps tank so I switched to the Ushio 10Ks, I was expecting to trade a little intensity, the ushios arent quite as intense as the iwasakis, for a bunch of coloration, its said that bulbs with higher kelvin temps will increase sps coloration because the light is more blue, boy was I disapointed, it hasnt been quite a month yet since switching so I really cant say for sure whether the sps will color up more under the new bulbs, but I can say the iwasakis actually were more white than the ushios, the tank color looks almost identical, and I recently ran the ushios without the actinics to see their true color and they are really yellow
Wish I would have went with 20Ks now
As mentioned I also run ushios on my 55 and they have the same yellow coloration, but they are far more intense than the 55K. The point is you should consider more things than spectrum when choosing a bulb. Spectrum may play a role in photosynthesis, but its usually in terms of color, not growth, what your looking for is a more intense bulb. The ushios would be the best bet in your situation, they dont differ much in color from the 55, but they are far more intense in terms of PAR. 65k is also a good choice, but as mentioned iwasaki is the best and its not available for 175s. HTH
