When adding substrate/ ground cover to your tank, always have at the very least 3 inches of substrate at the bottom of the tank. Not only does it prevent the tank from getting too dry by holding in the moisture, but some tarantulas dig and hide in the substrate to provide warmth for themselves if needed. If you find your tarantula doing this, i recommend at least 5″ of substrate as this allows ample space for you tarantula. Always make sure that after adding substrate, measure from the surface of the substrate to the peak of the enclosure. The length should be no longer that the width of your tarantula from one leg to the other. Falls from greater lengths than these could injure your tarantulas abdomen and fatal falls could potentially kill it.
For substrate / ground cover I recommend the following (links open in a new tab),
When deciding what type of substrate you want to go with, it really boils down to preference. Sand is generally not recommended, the only exception would be using it as a small part of a mixture that contains mostly soil. Things like gravel, perlite or corn cob or any other type of small animal bedding, especially cedar shavings should always be avoided. This is why the above ones are the only ones I would recommend. At TarantulaPets, we have experimented with many and found that this works the best for the Tarantulas that we recommended here for beginners. Most of which are burrowers and are not a fan of high humidity. If you choose to go with something else, please ensure that it is chemical and pesticide proof, this can potentially prove to be fatal for you pet tarantula.
It is recommended that you pour some water down the side of the tank and have the bottom depth damp while the surface that is in contact with the spider remains dry. Ideally, the slightly damp corner should be where the water dish is while the other end of the tank is dry. Over time this water will evaporate and provide beneficial humidity. Be mindful to replenish as needed taking care to not saturate the substrate, all this of course varies with different species of tarantulas (check out our recommended tarantula section for specific details).