Reefbrite is a strip of LEDs. The reefbrite xho has twice the number of LEDs than the standard strip. They come completely assembled with mounting brackets and everything you need. With reefbrite strips you can mount a 4' strip in a 4' canopy.
The t5 retrofit consists of the mounts to hold the bulbs, the reflectors and the ballast for power. You can screw the mounts into the top of your canopy. It does take a little more work than a store bought fixture but will not cost as much. You can use t5s that are they length of the tank but if ATI recommends one foot shorter than the length of the tank than that's what I do. I've been assuming you have a canopy over the tank but it you do not have a canopy then a fixture like the one in post 25 will work great and if you add a reefbrite strip to a fixture then you would want one that is the same length as the fixture so you can mount the reefbrite to the fixture.