I think what you probably read had to do with cyanobacteria, which is NOT algae. When lamps degrade, their spectral output often shifts more to the red into a range that is favorable to cyanobacteria.
Unfortunately you have chosen a specific Kelvin rating -- 6700K -- that was picked out of thin air to describe certain compact fluorescent lamps that emit in the 450-460 nm range. These are actinic fluorescents that do not emit in the so-called "true" actinic range of 420 nm, often marketed as "true 03" actinics. I hope I'm not confusing you even more but there are, generally speaking, two different types of actinic, or blue, fluorescents: those that emit at 420 nm (called true actinics) and those that emit in a range near 450-460 nm. The people who manufacture compact fluorescents that emit in the latter range have chosen 6700K as the Kelvin rating for their lamps. Their lamps are NOt 6700K but that's the way they are marketed.Well, what would that kelvin range be considered? Like 6700K?
So when you say 6700K or lower, you are confusing the issue. You should have said 6500K or lower. However, to answer your question, 6500K is fine for a reef aquarium as far as most corals are concerned, especially SPS that are found in nature near the water's surface. The only problem with 6500K (or 6000K) is that it appears a little too yellowish to most hobbyists unless it is supplemented with lots of actinics. To make matters even more complicated, not all 6500K or 6000K lamps are the same. Some are more yellow in appearance than others. Which brings up another topic: you can't rely on Kelvin ratings at all when it comes to appearance because the various manufacturers lie about the true Kelvin ratings of their lamps. Many of them choose Kelvin ratings based on marketing without regard to the true Kelvin ratings of their products.
There is no one-size-fits-all answer to your question. A lot depends on what you prefer personally when it comes to the appearance of your tank. Some people prefer a more bluish appearance than others and are willing to trade PAR and higher growth rates for the blue appearance that they find attractive. You will have to research this topic a little more and decide for yourself what you like. Not what others like but what you like. Then you will have to decide if that's what you want or not.What would be the best K ranges for a reef system?
If you're talking about T5 HO lamps, you can mix them to your heart's content and come up with just about any appearance. If you're talking about metal halides, you might decide that 10,000K lamps (for high growth) supplemented with actinic fluorescents give you a combination that you can live with both for optimal growth and a pleasing appearance. If you can tolerate 6500K Iwasaki halides and squeeze in enough actinics to "fix" their appearance, then you will have even more growth.
The bottom line is that no one can answer this question for you. You have to answer it yourself.
P.S. -- Some 14,000K halides are bluer than some 20,000K halides. Iwasaki markets a halide lamp as a 50,000K lamp. That's hilarious. It's not as blue as most 20,000K lamps. And all of the 20,000K lamps are not really 20,000K, they're simply "blue." They're too monochromatic to have a real Kelvin rating. That's why Radium says that their lamps are "blau" (German for blue) and not 20,000K. We call them 20,000K but Radium calls them "blau."



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