What the substrate needs to do
Use a clean soil-style substrate for Cyriocosmus perezmilesi that can hold its shape without becoming hard, dusty or permanently waterlogged.
For Cyriocosmus perezmilesi, the substrate in a Bolivian Dwarf Beauty enclosure should support normal movement and retreat building. Heavy decor must not be able to collapse if the spider digs underneath it.
Depth
Bolivian Dwarf Beauties are fossorial, so give Cyriocosmus perezmilesi 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 perezmilesi. Small slings and fossorial individuals often use proportionally more depth than a large settled terrestrial adult.
Moisture
For Bolivian Dwarf Beauties, use moderately moist lower substrate with good ventilation and a small water dish when size allows. 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 perezmilesi 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 perezmilesi does not collapse immediately. The upper surface can remain looser where appropriate.
For Cyriocosmus perezmilesi, keep rocks, heavy cork and ornaments in a Bolivian Dwarf Beauty enclosure low and secure so excavation cannot undermine them.
When substrate needs replacing
Spot-clean waste and prey remains from the Cyriocosmus perezmilesi enclosure. Normal digging, webbing or rearrangement by the spider is not a reason to replace clean substrate.
For Cyriocosmus perezmilesi, replace all substrate for a Bolivian Dwarf Beauty only when it is fouled, contaminated, persistently mouldy or no longer structurally usable.