S.O.S.
No it is not necessary... some of the time.
Frankly, corals (or more precisely, the dinoflagellate guests we call zooxanthellae in the corals) can adjust to a range of lighting conditions, else the coral will seek to substitute the current crowd of zooxanthellae for another, more suitable kind.
You asked if you could get by on MH?
Depends on wattage, distance from tank and size of tank ---but then the nature of your question seems to focus on
quality or
spectrum of lighting as opposed to magnitude.
The best thing to base such lighting requirements on is where the corals were collected --shallow water specimens are quite used to a lighting spectrum with more yellow and red in it. The lighting at the LFS where you purchased the coral is a less reliable, but still valid source of ideas. If the coral's been there awhile and seems to enjoy the shop lights, then find out what lights are actually being used.
So long as you provide sufficient light (and again, the magnitude can be guessed at based on LFS conditions and whatever is the prevailing opinion online these days, hehe), you've got half a foot in the door.
KEEP IN MIND:
It ain't just light
Few corals can get by on just the photosynthetic production of their guest zooxanthellae. Dem corals HAVE got ta eat.
What DO corals eat?
Bacteria and motley protozoa growing in their mucus --even particulate organic debris (including the poop of other creatures, haha) trapped by said mucus is one thing that can automatically be supplied by most any reeftank.
Plankton can be captured right out of the water. Live rock and Live
sand beds harbor the vast horde of critters and plants that generate such plankton. Granted, it can take awhile for a reeftank to settle down to significant plankton production, and this is partly why 'refugia' are so useful. ((Some corals actually have large-enough polyps to enable hand feeding with small bits of shrimp.))
So, good lighting, and a riot of small-critter biodiversity --
that will take care of your coral's food needs. From there you're mostly left with water quality and chemisttry, temperature, and disease/predator control --to ensure the health of your coral.
hth,
horge
Emperor of the unKnown Universe
Grand Poobah of the Deep
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