I used 10 watt multi chips for DIY fixture. There are a few more builds out there using them these days & might be worth looking into. One of the main reasons I did this is there are individual heatsinks with fans for them allowing me to keep the weight down. Downside is more wiring for all the fans.
There's an extensive thread on RC about the multichip LEDs. Look for 'minimalistic' in the title. It's a long read though. Link to my build follows:
http://www.canreef.com/vbulletin/showthread.php?t=82206
I used a total of 27 ten watt chips on 3 rails. The outer rails are tilted 10 degrees inward. Dimming is manual so you'd have to use a different driver/power supply configuration than I have. You could cover your narrow tank with 2 rails nicely.