Hi,
The nicest advantage with the metal halide is that it generates the light from a single small spot (the arc within the center of the bulb) which in turn creates a nature like "light rippel" through the water column, if there is turbidity on the water surface from the waterpump outlet. You will loose this effect if the tubes are on at the same time.
Also some halide bulb generates a very natural appearence with sharper shadows and also some bulbs have very nice colors on their own. But not all bulbs are equal and I have experienced two new bulbs from the same manufacturer and the same type, where one gave greenish hue and the other a slight bluish...... so it seems that one needs some luck when buying a pair to get them matched. Also it means it is quite expensive to "try and error" your way to find a bulb with the color of your liking.......
Most often I have seen a metal halide in combination with blue flourecent tubes that have their spectrum concentrated in
a peak where the zooxantelle organisms photosynthezise within some corals......this application is suitable for
salt water aquariums but in my opinion is a bit to blue for freshwater, unless you want to simlate the light in some spring rivers in Florida (Crystal river, Rainbow river).
Cheers
