Setting up a saltwater tank involves the decision of what type of gravel (substrate) to use. Saltwater tanks have fewer options than do freshwater tanks, as aquarists want to re-create a natural environment. Saltwater tanks depend in part on the substrate you select for proper water balance and chemistry; the materials used are calcium-based and this mineral is vital to the health of the tank. Marbles, colored gravel and the like are not suitable for the saltwater aquarium. Here are a few choices for your saltwater tank.
If your goal is to set a reef tank full of live rock, a bare bottomed tank is an option. The rock can be set onto the glass bottom of the tank and over time you may see some sand form as the tank ages. The live rock and coral serve as the calcium source in the tank.
Crushed coral is the most common choice and the most economical. You will need about 1 pound of rock for each gallon of actual water that is in your tank. This will vary based on the type of filter system you are using; often you can use far less. Crushed coral is a good choice as it is easy to maintain and you can use an aquarium vacuum to clean the tank floor without having the gravel siphoned off. It does have a tendency to become covered with algae, which is not attractive, but a good vacuuming usually decreases the effect.
This is another very attractive choice for the bottom of the tank but a little more expensive. The tank has a natural appearance and, because of the shells natural coloration, algae formation is not as noticeable. The amount you need will vary with the species you are trying to keep, your filters and any supplements you are adding.
Sand will give you a natural reef appearance, but it has special maintenance considerations. Sand cannot be vacuumed as can the other substrates because it will be removed by the suction tube. If you have a sand bottom, you will need to insure that you keep a few bottom feeders to keep waste and food particles from accumulating. You will also have to avoid the use of any under-gravel filtration systems as the sand will clog the filter and impede the flow of water. A dramatic display can be created by the use of black sand that really shows off the colors of marine fish.
All gravel and sand needs to be washed and rinsed thoroughly before adding it to the tank. After water and salt mix is added expect the water to be cloudy and give it a day or two for the small particles to settle.