Quote Originally Posted by MizTanks View Post
What do MH and LEDS offer that other light sources don't?
It's all to do with spectral output and relative intensity. Each light source creates light in a slightly different way. Some light sources can create light more efficiently in some wavelengths better than others. That doesn't mean that any one light source is worse than others. It just means that some lights are more efficient. As long as the lights can provide the corals what they need at the right intensity, then anything can be used. PC's have typically been considered the bottom rung of the lighting ladder, but that's only because manufacturers have been always considered them a cheap, and less critical technology. As a result, the phosphors used have been sub-standard compared to T5, even though they are basically identicle. Companies like Current USA, UVL and Hamilton have been using T5 phosphors in their lamps for a while now, and the difference in output over your standard PCs is considerable. Now, PCs will never be as powerful as T5HO because of the geometry and reflector issues.

Is it their high par, high kelvins?
High PAR only does you good if the light is in the right wavelengths for what you are growing. Obviously, most lights targeted for reef lighting are set up the right way, so more PAR is better. But, there is a trade off with color. If you like deep blue tanks, then you don't always get the option of having super high PAR. For the most part, as long as you don't venture outside of what is considered the norm for reef lighting, it all works.

What does *watts* have to do with coral growth is anything? i.e. 150~250~18~24, ect ect.

Thanks again Evil

Jamie
Watts has very little to do with anything. It used to be a simple metric to use back in the day when PCs were still very popular to guage the relative performance of the fixture over a certain tank. It was the old watts per gallon rule. This all gets back to efficiency. T5HO, MH, and LED all produce more light per watt, so putting the same wattage of light over the same tank now means you are putting more light into the same area. LEDs were really the technology that put the last nail in the coffin for that old rule, as LEDs can produce tons of PAR for very little energy used.