Choosing a Substrate
Substrate choice follows from behaviour and humidity, not from what looks good in a shop display. Some of the most widely sold options are the ones to avoid.
Start with the animal, not the aisle
A fossorial skink that spends its life under the surface needs several inches it can tunnel in and hold a burrow. An arboreal gecko will barely touch the floor and the substrate is essentially a humidity buffer. A grassland tortoise needs something that will not cake. These are different problems with different answers, and a single 'best substrate' recommendation usually means the writer had one species in mind.
The ones to avoid
Cedar is the clearest case: its aromatic oils are an irritant and it does not belong in a reptile enclosure. Calcium sand, sold specifically to reptile keepers and marketed as digestible, is a long-standing impaction concern. Fine dusty substrates are a respiratory problem for the animal, particularly in a poorly ventilated enclosure.
When plain paper is correct
During quarantine, while an animal is being treated, and whenever you need to see what is coming out of it. Paper towel and newspaper look like nothing, and that is their advantage: stools, blood, mites and parasites are all immediately visible. Aesthetics can wait.
Substrate is also a humidity decision
A deep moisture-retentive substrate is one of the better ways to hold humidity without closing off ventilation. The same property is a liability in an arid setup, where damp substrate held against the animal is how scale rot starts. Decide substrate and humidity together, because each constrains the other.
Common questions
What substrate is bad for reptiles?
Cedar, because its aromatic oils irritate. Calcium sand, which is a long-standing impaction concern despite being sold as digestible. And any very fine, dusty product in a poorly ventilated enclosure, which becomes a respiratory problem.
Is sand safe for bearded dragons?
Loose sand, and calcium sand in particular, carries an impaction risk that is well documented in the hobby and avoidable. Many experienced keepers use a firmer soil and sand mix that holds a burrow, or non-particulate options, rather than loose sand alone.
What is the best substrate for a snake?
It depends on whether the species burrows and what humidity it needs. Paper is correct during quarantine. Beyond that, a moisture-retentive substrate suits humid species while an arid species needs something that will not stay damp against the animal.
How deep should substrate be?
Deep enough for the behaviour. A burrowing species needs several inches that will hold a tunnel rather than collapse; an arboreal species needs only enough to buffer humidity. Depth is a function of the animal, not the enclosure size.
Sources
Related reading
all husbandry fundamentals · common mistakes · finding a reptile vet