As far as LEDs, count the number and type that make up the fixture. They need to be at least 1 watt for each LED and a mixture of white and blue if coral growth is what you want. Most panels big enough to equal a 300-400 watt metal halide needs 120-150 1 watt LEDs in the panel. Fewer if the new 3 watt LEDs are used. They get hot collectively and need a proper heat sink. Think computer gear rather than conventional lights, as they draw far less power and project far less heat into the water. But they usually require fans if they have the proper amount of PAR output.
In my humble opinion, fluorescents are fine for smaller tanks that are shallower, but metal halide is usually needed to get a high PAR rating in a deep tank for coral growth. LEDs can do both jobs, light a small tank without heat buildup and when properly ganged up with enough total wattage, provide brilliant light to the very bottom of even a deep tank. They are not more expensive when you compare the life span of a LED to even a florescent high output lamp. They blow away metal halide as they cost $80 apiece and rarely last a year. They are the future of lighting in general.
I now sound like a LED salesman...
