For what it's worth, driving XML's at 700-1500mA is NOT under-powering an XML. True that it's not driving it at it's full performance, but it actually is running this particular LED in it's most efficient drive current range (700mA), where it hits 160 lumens/watt.
It's also true that the XPG's are cheaper, nearly as efficient (although NOT at 700mA), and quite beautiful.
The XML will give you a touch more light at 700-1000mA than the XPG but not a lot more.
My plan was to take the extra time to drill and tap the heatsink to allow me to replace LEDs if I do not like the color combination.
I will be using 2 rows of 9 LEDs. Any advice on how wide of a heatsink I should use? I was thinking of getting a 6" x 42" heatsinc. I am also going to go by my local scrap yard to look for some alternatives to using a pre-made heatsink.
Drill and tap, very good idea.
If your tank is 13" wide, then you only need a 6" wide heatsink, with the LEDs placed about 1/2" in from the edge in two rows.
Running LEDs at 700-1500mA demands some pretty thick aluminum with fins (and possibly fans). I personally like the Heatsinkusa.com offerings but they are a bit overkill and not cheap.
Lots of folks are using aluminum C-channel, but it will vary in thickness from one style to another---- get stuff that is AT LEAST 3/16" thick. Preferably 1/4" thick.
You really need the thickness/mass of that aluminum to allow for good heat sinking from the LEDs.
Computer CPUs and car amp stuff will work well as long as you can figure out how to mount it