rapid led ? for my tank

[charliethetuna88]

I'm thinking about switching over to the led world.I wanna use the rapid led retro fit kits ,i have 2 tanks that i will be doing one Is I think around a 30 cube its 18x 20x 18 .how many cree 3 watt bulbs would I need .i kindalike the color to favor the 20k spectrum .also how many for my 120 gallon the measurements on that is 48x 24x24 I have a centre brave so I would like to do 2 separate units on that tank