Hi,
basically, the amount of light you will need depends on the critters you are trying to keep. Basically, most corals and many other organisms such as anemones, clams, so gorgonians, etc are photosynthetic, and need light to grow.
In a reef tank, you are trying to simulate as close as possible the natural conditions which would be encountered on a reef. THis includes lighting.
If you look at the amount of lighting on a coral reef, you will get an underwater lux reading of about 15 000-70 000 lux and a depth of 5-20m (16-65') (Jauch, 1988), which is where most of the animals we keep in our aquarium come from. In order to keep them healthy, we must try to replicate this as closely as possible. However, 2 NO fluorescent tubes will not give anything like this reading.
I know many people who have a 400-watt MH over a 29-gallon reef so that they can get reasonable amounts of light to the corals in their system. If you use more intense light, you will get much faster growth and stronger colours from your corals and other photosynthetic organisms. I was amazed at the difference when I switched from 150-watt to 400-watt MH's on my 180-gal. reef. I have no trouble growing Acropora and other SPS corals and the colours that they get are amazing. I bought a brown piece of Acropora from a LFS recently, and now under the 400-watt is has developed green tips and is growing rapidly.
Finally, I should add that the watts per gallon "rule" is just a rough guide, not a hard, fast rule. I know people who have 1000 watts of light over 55-gallon tanks (2 400-watt MH plus 2 VHO actinics). I have a little over 1000 watts over my 180, but again that's not because I set out to put x watts of light over the tank, it's because I felt that for the creatures I would like to keep I would like to have 400-watt MH's. Of course, that is not to say that if all you want to keep is star polyps and mushroom polyps that you should run out and buy 400-watt MH's, but that for most photosynthetic organisms natural light levels are better. An anemone moving around is usually a sign that he is not happy, and this may well be due to the lack of light.
Regards,
Tom
