<a href=showthread.php?s=&postid=10451771#post10451771 target=_blank>Originally posted</a> by der_wille_zur_macht
The info in that Krib article is good in theory, but poor in the fact that it's nearly 10 years out of date.
FWIW, it's very easy for powerful MH bulbs to produce illumination in our tanks that is higher than the intensity of light at the ocean's surface on a tropical reef. Go read up in the Lighting, Filtration, and Equipment forum if you want numbers to back it.
There are two big issues with letting (or promoting) natural sunlight in a reef tank. First of all, the spectrum is not going to be "correct" - either visually or photosynthetically. The fact that the light has passed through glass before it reaches your tank at home causes some of this, but also the fact that, on a natural reef, the light has likely passed through several feet of water (at least!) before getting to the corals and other creatures we keep. That shifts the spectrum quite a bit compared to light at the surface, and commercial reef lighting products have been designed to mimic that, rather than the spectrum of sunlight as it hits the water's surface.
The other big obstacle is that, unless you live in an extremely sunny and stable climate, the light will be unbelievably variable from one day to the next. This type of variation (along with the not-so-natural color spectrum) tends to fuel the things we don't want in our tanks (pest algae) more than the things we do want in our tanks (corals!). Hence, the rumors common in the FW world about natural sunlight fueling algae.
Oh, and another disadvantage, linked somewhat to the variability factor. Natural sunlight can heat a tank to sky-high temps in a short time if it is direct and intense. Most people with reef tanks have so much equipment dumping heat into the water already that this can be a huge problem.
Personally, I wouldn't worry about diffuse, indirect sunlight, but I wouldn't place a reef tank in a location that got much natural sunlight at all, unless I lived in SoCal or someplace else that's the same (sunny!) 99% of the time and I wanted to experiment.