t5, t8, t12 simply refers to the diameter of the bulb in eighths of an inch. The biggest, t12, is the least energy efficient, and I'm told that they don't last as long as the smaller ones. T8 is more efficent than t12, and t5 is the most efficient.
If you want to suspend the lights above the shelves, so that height is adjustable, make sure that you buy that kind of fixture. Some can be hung from chains and plugged in. Others have to be mounted and hard wired.
Adjustable height is desirable, so you can raise the height as plants grow. The other option is to use removable things on the shelves, under trays.
Karen
