First off, you must decide how many LEDs you are going to install, and how to install them. I decided to use a 18x3 grid of LEDs. For my 10g tank (20" long), I chose to start the LEDs 1" in from each end of the tank and a 1" space between LEDs. Your design will most likely vary. You also must consider the fact that resistors are needed when working with LEDs. I had this
site recommended to me and I found it to be VERY useful. You must know what voltage and current your transformer will put out. You also must have your LEDs chosen and know the voltage and current required by them.