Attractive decoration is of paramount importance to the presentation of any marine display. Although many of the choices are subjective, there are a number of practical consideratons such as how safe the rocks are, are they likely to affect the water quality and will they compliment the livestock.
 

LAVA Rock- is a natural volcanic rock, which is not recommended for the marine aquarium as it contains heavy metals and toxic substances.
  Safe lava rock make excellent homes for invertebrates and fish alike. It is light, porous, displaces very little water, and locks together well into safe structures. On the negative side, lava rock is very expensive, has dangerously sharp edges and is usually incapable of being worked. It has o pH buffing ability whatsoever and may even be slightly acidic.

LIMESTONE- A very hard rock of an alkaline nature and well suites to the marine aquarium. However, it does displace massive amounts of water.

SLATE- Slate is inert and may be used without fear of toxicity. It looks rather unnatural, is heavy and displaces a lot of water.

BARNACLE SHELLS- giant shells are very decorative and are convenient as they offer a place to live o the smaller fish and the invertebrates

SANDSTONE- decorative sandstone with attractive sand blasted shapes and varying colours are an interesting choice for people who want something unusual. However, it will discolour badly as micro-algae colonize the surface.

Other decorative items- Ceramic jugs and vases can be used to create special underwater effects and are safe to use.
  Plastic to7ys and novelty tend to look fake and may not even be safe as some plastics and dyes are toxic underwater.