Live rock

[rossb]

Finally you need some silica sand (the cheapest you can find).

Much discussion on this. Once again claritiy is important. The cheap sand is for the sand mold. The more expensive silica sand is to mix with the oyster shells, and can be replaced with crushed coral.

On the ingredients, as Steve notes, one must be careful...
It is also aweful to buy some live rocks infested with nasty beasts that eat your shrimps and crabs.