There are a couple recommended substrates for tarantula enclosures and there are things you should never use. Good Substrates Cocofiber - is one of the best substrates to use. It holds moisture very well and your tarantula will like to walk on it. Tarantulas also find it easy to dig their burrows in. It is found in most pet stores in a dehydrated brick. 1 brick makes enough substrate for several medium sized enclosures. Half a brick will fill a 13x9x2 baking sheet to the top. Take the brick out of the wrapper and cut it in half then add about 6 cups of water and let it soak up the water. Keep adding water and allowing it to soak in until it is easy to break up. Once it's broken up squeeze out the extra water and place it into the enclosure and pack it down. Peat Moss - is a good additive to your substrate but not as the only substrate in an enclosure. It can also be used as decoration in an enclosure. Search youtube.com for tarantulaguy1976 and find his back drop tutorial as an example. You will see his use of peat moss in his enclosures. Vermiculite - is a good substrates to use. It holds moisture well and your tarantula will like to walk on it. I have not used it but have heard and read that it is good to use. Bad Substrate Bark chips of any kind - Tarantulas do not like the feel of bark chips on their feet and will climb the sides of the enclosure. You do not want your terrestrial tarantula to climb, they are fragile and can severly injure themselves if they fall from even short distances. Cedar or pine chips will harm your tarantulas health, the sap from the pine will coat their hairs making it unable for them to molt. Cedar is not liked by most insects or spiders. Bark chips also hold moisture poorly. Your tarantula will also be unable to build a burrow of it's own and most terrestrial tarantulas do dig burrows. Aquarium Gravel - Tarantulas do not like the feel of bark chips on their feet and will climb the sides of the enclosure. Moisture does not soak into gravel but sits at the bottom of the enclosure. Your tarantula will also be unable to build a burrow of it's own and most terrestrial tarantulas do dig burrows.