What the substrate needs to do
Use a clean soil-style substrate for Cyriocosmus aueri that can hold its shape without becoming hard, dusty or permanently waterlogged.
For Cyriocosmus aueri, the substrate in a Peruvian Dwarf Tarantula enclosure should support normal movement and retreat building. Heavy decor must not be able to collapse if the spider digs underneath it.
Depth
Peruvian Dwarf Tarantulas are fossorial, so give Cyriocosmus aueri deep, compactable substrate that can hold a burrow. Depth is part of the enclosure design, not an optional extra.
Match substrate depth to the current size of Cyriocosmus aueri. Small slings and fossorial individuals often use proportionally more depth than a large settled terrestrial adult.
Moisture
For Peruvian Dwarf Tarantulas, use lightly to moderately moist lower substrate with ventilation; never waterlogged. Add water in a controlled area and judge how the enclosure changes over several days instead of trying to keep every centimetre equally wet.
Substrate moisture for Cyriocosmus aueri should work together with ventilation. Persistent condensation, a stale smell or widespread mould usually means the setup is staying too wet.
Compaction and stability
Press the lower substrate layers firmly enough that a retreat or burrow for Cyriocosmus aueri does not collapse immediately. The upper surface can remain looser where appropriate.
For Cyriocosmus aueri, keep rocks, heavy cork and ornaments in a Peruvian Dwarf Tarantula enclosure low and secure so excavation cannot undermine them.
When substrate needs replacing
Spot-clean waste and prey remains from the Cyriocosmus aueri enclosure. Normal digging, webbing or rearrangement by the spider is not a reason to replace clean substrate.
For Cyriocosmus aueri, replace all substrate for a Peruvian Dwarf Tarantula only when it is fouled, contaminated, persistently mouldy or no longer structurally usable.