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Icefire- You need to check your specs on the R2 again... At 800mA, you are still getting 95lm/w. If they were luxeon K2's, then you would be correct with that sharp of a droop in efficiency vs current. The new Cree's have a shockingly minior droop curve to about 800mA, then from 800-1600mA, it falls off like other LEDs. Note, 1600mA should only be tried by experts.
You are also way off on your lighting needs estimations. You only need 3x250w MH's on your 120 becasue the typical MH reflector is able to get less than 50% of the bulb lumens into the tank. Also, aquarium spectrum MH bulbs are around the <70lumens/watt range.
Due to the unique ability for LEDs to actually get a large amount of the light produced INTO the water (rather than making for a very bright warm canopy like MH), they only require making about 60-70% of the same bulb lumen numbers to match underwater par levels.
Due to CREE, they are finally able to make that light at reasonable emmiter efficencys.
You simply do not need 500W in LEDs. At 800mA each (roughly 2.5-2.8w depending on the indivdual chips Vf) that would mean 180 LEDs each making 250lumens. You would be generating about 45,000 bulb lumens, and likely getting 35,000lumens of those INTO the water. Most aquarium spectrum 250w MH can't get 10,000 lumens into the water even with lumenarc reflectors. Hell, most 20k 250w MH bulbs don't even make 10,000lumens at the bulb!
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