I would start by looking at the specs commercial units for example.
According to Aquaillumination 75 watt led system = a 270 watt mh
Then there are many design issues you need to define. What LED am I using and how does it
compare, Do I need red and green LEDs, lenses etc..
http://aquaillumination.com/sol/specs.html
http://aquaillumination.com/sol/
LEDs
Array:
8 Triple Lenses ( (2W x 1B) * 8) = 24 LEDs total
White:
Quantity: 16
Cree: XPGWHT-L1
Correlated Color Temperature (CCT): 6500K
White LED Wavelength
Blue:
Quantity: 8
Cree: XPEBLU-L1
Dominate Wavelength: 470nm
Blue LED Wavelength
EcoTech Marine Radion XR30w LED Light Fixture Information
http://www.marinedepot.com/EcoTech_...ures-EcoTech_Marine-EM3111-FILTFILDTN-vi.html
LED
8x Cree XP-G Cool White LEDs run at 5w each
8x Cree XP-E Blue LEDs run at 3w each
10x Cree XP-E Royal Blue LEDs run at 3w each
4x Cree XP-E Green LEDs run at 3w each
4x Osram Oslon SSL Hyper Red LEDs run at 3w each