MrHarvard
"ship it"
Couple things...
XT-E's at 700ma have a forward voltage of 3v. So you could run 15 per LDD at 700ma.
The LDD require PWM for dimming. The Apex doesn't support PWM, you'd need some sort of converter to go from the Apex to the LDDs to change the output to the correct voltage for D.
Pedro, you may even want to consider using Luxeon M 12w blue leds rather than using the cree 3w. I used them along with the 3w hyper violets on my current fixture. I'm not sure of how your going to lay your leds out but these can save you space and they pack a punch. If you want to see pics of my fixture let me know or if you have questions. If you want to use your apex to controll them you can buy the converter and drivers from Stevesleds that allow you to hook them up to an apex. Here is the link.....
http://shop.stevesleds.com/Aquarium-Controller-Interface-Harness-for-Steves-LEDs-Drivers-harness.htm