Watts per gallon, T5 vs MH

I'm looking at metal halide lighting but am noticing that the wattage they put out is much higher that what my T5's do and more that what I was told is recommended which could end up in melted corals. Example, a MH fixture that could put out 716 watts of light which is 9.54 watts per gallon. Would this be too much light for a reef tank? I want to be able to have whatever I want in the tank so I need all the light I can get but how much is too much? They have one that has 408 watts (5.44 wpg) and 608 watts (8.10 wpg). I'm leaning towards the 608 watt.

I currently have 716w MH/T5 over my 75 and it's not too much. What you have to think of is that it's only 500w of MH and 216w of T5 (and the T5s don't have individual reflectors so it won't be as effective for lighting). I use the T5 in my fixture mainly as accent lighting to get the colors of my corals the way I want them to look.

Watts per gallon is not a very good means of measuring actual light in your tank. 100 watts of PC light is not as bright as 100 watts of MH, while 500 watts of MH might be less light than 150 watts of LED, etc.

Without a par reader, there's really no telling how much light you're getting. All you can really do is either place the corals near the bottom and move them up towards the light as they adapt, or place the corals where you like and lower the light over them gradually, while watching them for any signs of improvement, distress, etc.

Your best option is too just see what everyone else is using for your size tank, and after you choose a set up, making sure you give the corals some time to adapt while watching for changes.

Oh, and when you set your lights up, you'll want to incorporate some way to adjust their height, either by a hanging kit or building some sort of provision in your canopy.

Watts per gallon is pretty much a useless measure these days, with things like T5's with individual reflectors and LED's becomming commonplace. Whats more important is how deep your tank is, and making sure that you have lights that will cover not only the horizontal span of your tank, but its depth as well

As far as being able to melt corals - its certainly possible - I melted some green slimer Acropora after switching to LED's because I did not properly acclimate the coral to the light. As long as you acclimate existing corals to the new lights (by moving them lower in the tank, or by shortening the photo period temporarily) You should be fine