They come with hanging hardware. You can build a rack to hold them over the tank if you wish. This is where you get a little creative. Yes, 2 are perfect for a 75.
@Tategrant- You don't necessarily need 120 watts. It depends on the type of LEDs, the optics, and placement also. These are a suitable plug and play option. You can turn the potency up or down as needed, and to tweak the amount of white and blue.