Those look like great LEDs for an aquarium, but I notice they are for a maximum of 700 mAmps, and the Meanwell driver you are getting will drive them at 1050 mAmps, maximum. You will need to use an ammeter to adjust the driver down to 700 mAmps before using it.
Judging by my LED light, using 130 lumen LEDs vs your 220 lumen LEDs, you might get about 80 micromols of PAR at 18 inches. I think I would try to evenly space the LEDs, about 3-3.5 inches apart, leaving the middle of the fixture blank because of the cross brace on the tank. And, I think I would use all 30 LEDs, in two 15 LED strings. Meanwell drivers have an adjustment screw for the output voltage, so you should be able to raise the voltage enough to accommodate 15 LEDs. You could then use 3 rows of LEDs, 10 in each row, spaced 3.25 inches apart, with about a 6 inch gap in the middle. This would be close to my setup, except with about twice the PAR.
I don't think you need a finned heat exchanger for this. Look at getting aluminum channels, 1/8" thick aluminum, one channel per row of LEDs. In fact, this would be a good time to do a lot of thinking about how you will house the heatsink, or whether you want the heatsink itself to be the housing, and how to hang it. Ebay has some stores that sell aluminum channels - Online Metal Supply is one.