Well the 50/50 is a combined daylight that also has a lot of energy in the 420 nm range, said to help with corals photosethesis process. Unfortunatly 20 watts is not much light energy, no matter how small a tank its on.
One of the reasons the old, oft quoted watts per gallon rule is not really valid is that it doesn't deal in PAR, Photo Active Radiation, a measure of light energy in wavelengths that are usable by organisms, in our case coral.
Going strictly by the watts per gallon rule, if you had say a 29g std and mounted a 150 watt regular household bulb over it, you would have approx 5 watts per gallon and should theoretically be able to keep a majority of photosynthic organisms. not so unfortunatly, the lumen output is fairly dismal and the color spectrum is all wrong for photosysnthesis.
Now replace the standard household bulb with a 150 watt Metal in 6500K or higher Kelvin rating, or a 150 watt
double ended HQI (Halogen Quartz Iodide) with their specialized ballasts and filaments and now you are getting some serious light output while using the same 150 watts of power , I am thinking something along the lines of 10x the light intensity in a spectrum that corals can use. Figures arent exact but you get the point.
With metal halide and the other high output lights the ballasts that make them work add to the cost but they are commonly available in the trade and you can put together lighting systems for fairly cheap if you are willing to do it yourself. Ready made always costs more, but the specialized bulbs that have the right combination of inert gases, phosphoring elements and the other factors involved in producing a specific color temp are the spendy part, normal lighting for illumination does well with color temps in the 3000-5000 º Kelvin rating so they are mass produced and fairly cheap. Specialty bulbs with the 10K and higher color temps are made in small numbers for a specialty market hence the higher cost
The infamous Iwasaki 6500K bulb is actually a mercury vapor lamp that will run on a metal halide ballast of the correct voltage, since they are multi use they tend to be about the cheapest High Output bulb thats usable in reef applications, esp if you find them from an electric supply rather than an aquarium supply source
