Every form of substrate has its pros and cons. The choice of substrate is often a matter of personal preference (aesthetics) and practicality (size of collection). Some prefer a minimalistic approach and feel it is easier to clean and maintain a more sanitary environment. Others like a more naturalistic approach and feel that mulch or a similar substrate is more typical of that in the wild. You will likely experiment with different substrates before you settle on the one that is best suited for you and your collection. But as you do, avoid wood shavings or bark such as cedar or pine since they can have strong odors and might irritate the mucus linings.
Absorbent Pet or Bed Pads
Absorbent pads work great for retaining moister and will help maintain elevated humidity. Also, some can be cut to size. But their higher cost can be limiting if you are keeping a larger collection. They can be found at most big-box retail stores.
Many keepers with large collections use newspaper as a substrate. It is cheap, easy to get, and is simple to remove/clean. The main drawback with newspaper is it can ink both the enclosure, and roaming animals within the enclosure. This is especially true with white materials (PVC) and yellow neonates. But this is purely a visual issue since ink is soy based and does not pose any health risk. Because newspaper dries out quickly you will likely need to spray more often to maintain an elevated humidity level. And it is not good at masking the odor of fresh feces, but this might give an incentive to clean more often.
Like newspaper, paper towel is a popular substrate for keepers with large collections. It is commonly used in tubs for its practicality; it is easy to clean and holds humidity well. However, caution should be used, especially when feeding. Although it rarely happens, ingesting a large amount of paper towel can lead to severe complications and often death.
Cypress mulch is popular among GTP keepers since it is readily available, holds humidity well, and tends to mask odor from defecations. It is visually appealing, does not rot or mold easily, and can be spot cleaned. Many keepers using mulch spot clean when necessary, and do a full substrate change 3-4 times each year. But while mulch is aesthetically pleasing, it does have its drawbacks. For example, mulch can become lodged in the mouth/gums from miss-striking a food item, which could lead to health complications. While spot cleaning eliminates the solid feces and urates, the remaining waste fluids will be absorbed. Also, mulch can harbor bacteria and fungus if kept too wet. It should also be noted that mulch often attracts gnats. Finally, it has been brought to the attention of the MVF community that harvesting cypress mulch is harmful to the environment since it is implicated in the destruction of wetlands. Do the research and make an informed choice.
Orchid bark has similar characteristics of cypress mulch.
Eco-earth or Similar Products
These substrates hold moisture well and look great in a more naturalistic set up. However, specialty substrates are expensive when dealing with many cages. Also, substrates that are very fine in texture, such as peat moss, coconut fiber, etc. can get lodged in the nose, heat sensing pits, and gums. This is especially true for roaming males and unsettled pre-lay females in a nest box. If left undetected, impacted substrate will likely lead to health issues.
Utilizing a substrate of standing water is not recommended due to the likelihood of developing unsanitary conditions.