Whether or not to put substrate or sand bed in your saltwater reef aquarium will depend on your preference. However, having a substrate or sand bed in an aquarium will have the following benefit:
1. It will make you tank landscaping to look more beautiful and attractive as a combination of sandy substrate with live rocks will create a more natural look or environment for your tank.
2. Having a sand bed will improve your tank biological filter de-nitrification process compare to having only live rocks. However, to achieve this, your substrate thickness or depth should at least be about three to four inch.
For more information about the benefit of substrate or sand bed in a saltwater reef aquarium, click on the link.