If the question is how much to keep live rock with out any corals then I would say it would be on the low end of the scale if there really even is a scale for it.
If your looking at softies and other such corals that are usually consitered lower light then it would be on the lower end of the scale. If your talking about looking at some of the hard stonie corals then it would be to the upper end of the range.
If yoru looking to go SPS
and clams then shoot for something above the upper range.
One reason WPG
is not the best factor is that only florecent lighting will cover the entire tank. If you have MH
lighting and a tank that is longer than 2' then your not going to get the full wattage over the entire tank. For example if you have a 4' long tank and you put a MH
bulb in the center you will have 1' on either end that is somewhat light with less light than the center 2' area.
WIth a watts per gal
calculation if you have 1 175W MH
bulb on a 55 gal
tank that would equate to 3.18Watts per gal
. But most of the light would be in the middle and very little would be on the ends. If you put 175W of florecent lighting on the tank the tank would be evenly light across the entire tank but the intensity at any one given spot would be lower than the center area of the tank that was light with 1 MH
So to properly light a 55 with MH
you would need two 175W bulbs to give even light plus you would probably go with some actinic lighting also. If you go with florecent lighting you would probably go with a couple VHO
bulbs or a couple power compact bulbs.