Naturally, you don't want a bare tank with just water and sand. Correctly placed pieces of rocks and coral will not only improve the appearance of your set-up but will also provide protection and hiding places which your animals need even in their natural environment.
Again, you must be extremely careful. After going to the trouble of keeping meals or other toxic substances away from the aquarium it would be disastrous to include rocks with soluble mineral content or other kinds of contaminants. Porous rocks can provide lodging places for all sorts of things including uneaten food and detritus. Use rocks like granite, marble or glass. It is possible to use natural rocks from the sea but chances are you may introduce unwelcome materials or visitors into your tank as well.
Coral is one of the most beautiful decorations for marine aquaria. It is the "skeleton" produced by hundreds of minute living animals closely related to the sea anemones and jellyfish. It is used only after proper curing. Boiling the coral in water followed by several rinsings removes all the organic matter, including the coral polyps themselves and other organisms using the coral as a home. Coral is very delicate and breaks easily so it should be handled with the outmost care. It also has many sharp edges that can cut or scrape your hand.
There are many different types of coral, practically all candidates for the aquarium. They range from the flat, oval or rounded Button Coral (Fungia) to the tree-like, many-branched Stags Horn Coral (Acropora). Brain coral is attractive but some part of the substrate is usually attached to the coral itself thereby making cleaning much more difficult than the high, branching coral which can be broken off above the point of attachment. Although 90% of cured coral is white, or a variation of white, some species have highly colored skeletons as the Organ Pipe Coral (red) and Blue Coral (blue). These make a pleasant contrast to the white corals. Due to the many tube-like openings of the Organ Pipe Coral where pollutants can accumulate it is perhaps wiser to place this species in the shadow-box instead.