Since the substrate is more than just the "stuff at the bottom of the tank", Choosing the Right Substrate for your aquarium is important. There are a number of different colors, types, densities and sizes of materials used for aquarium substrates. If you will be having a lot of water current in your tank, you will want a heavier substrate to keep it in place. If your substrate will be a significant part of the biological filter, porosity and surface area will be important.
To help you see what is available on the market and compare prices, here are some of the Top Saltwater Aquarium Sands & Substrates.
Live Rock plays an important role in a marine tank. Many marine animals, fish in particular, can be quite territorial. It is important to provide ample shelter or places where the animals can hide, sleep, and avoid potential problems with aggression from other tankmates in the confined space of an aquarium.
While a vast majority of marine aquariums utilize Live Rock, understanding the answers to the questions: What is Live Rock & Its Purpose? before you invest in Live Rock can save you a lot of money.