Choosing the right optics for your DIY LED build is very important. Optics are similar to high end reflectors for T5s. Sure you can put a T5 bulb in your canopy and it will shine light, but if you put a quality reflector behind it, it will make a world of difference.
When choosing your optics for your LEDs there are a couple things to remember.
1. Most of the light is in the first 50% of the degrees. For instance, a 60degree optic has most of its light in the first 30 degrees. It is not linear.
2. Choosing the optic is determined by the width and height of your tank including the height your fixture will be off the top of the tank.
Read more on choosing your optics here