In short: A bioactive terrarium substrate is a living soil layer that holds moisture, drains freely, supports plants and feeds a clean-up crew of springtails and isopods that deal with waste. A reliable peat-free recipe is 4 parts coco coir : 2 parts fine orchid bark : 1 part chopped sphagnum moss : 1 part horticultural charcoal : 1 part crushed leaf litter, laid 8–10 cm deep over a 3–5 cm drainage layer of clay balls separated by mesh, and topped with whole leaf litter. For a closed plant-only terrarium, a simpler 2 coir : 1 grit or perlite : ½ charcoal mix does the job. One 5 kg brick of coir expands to about 75 litres — enough for the substrate in a large 90 × 45 × 45 cm enclosure with plenty over.
Why coco coir is the base of the mix
The classic tropical mixes that terrarium keepers copy were built on peat: it holds water, stays slightly acidic and resists rotting. Coco coir does the same three things without the peat, re-wets far more easily when a corner dries out, and is what most UK reptile retailers now sell as "coco fibre", "plantation soil" or "coco bricks". On its own, though, coir compacts and goes airless when it is kept permanently damp. The other ingredients are there to fix that: bark and leaf litter keep the structure open, sphagnum adds humidity-holding without compaction, and charcoal helps keep a sealed, wet system fresh.
Use washed, low-salt coir with no added fertiliser, wetting agents or chemicals. Fertiliser in a sealed glass box means algae on the glass and a nutrient spike for animals that drink from the leaves; salt residues in unwashed coir are hard on frogs and invertebrates. Our bricks are washed to an EC below 0.5 mS/cm, calcium-buffered and additive-free — why buffered coir is different.
The peat-free bioactive recipe
| Ingredient | Parts by volume | What it does |
|---|---|---|
| Coco coir, expanded and damp | 4 | Water-holding, slightly acidic body of the mix |
| Fine orchid bark (pine bark chips, 5–10 mm) | 2 | Structure and air spaces; slow to break down |
| Sphagnum moss, chopped | 1 | Holds humidity without compacting; anti-microbial |
| Horticultural charcoal, small pieces | 1 | Keeps a permanently damp mix fresh |
| Crushed dry leaf litter (oak, beech, magnolia) | 1 | Food and shelter for the clean-up crew; fungal life |
| Optional: tree fern fibre or coco husk chips | 1 | Extra structure for very wet dart-frog set-ups |
Mix it in a trug until the colour is even. It should feel like a damp forest floor — springy, not sticky — and hold its shape when squeezed but crumble when poked. If water runs out when you squeeze, spread it out to dry for a day before use.
How to layer a bioactive enclosure
- Drainage layer, 3–5 cm. Expanded clay balls (LECA) or a purpose-made drainage mat. This is where excess water from misting collects instead of drowning the roots. Add a length of tube to one corner so you can siphon it out.
- Mesh barrier. A sheet of fine plastic or fibreglass mesh cut to size stops the substrate washing into the drainage layer.
- Substrate, 8–10 cm (deeper for burrowing species and for larger plants). Firm it lightly; do not compact it.
- Plants, wood and rocks. Rinse nursery plants' roots free of their compost first — houseplant compost carries fertiliser and sometimes perlite and pesticide residues.
- Leaf litter, 2–4 cm of whole dried leaves over the surface. This is the clean-up crew's home and it also holds humidity at floor level.
- Clean-up crew. Add a culture of springtails and a suitable isopod species, then give the enclosure two to four weeks to settle before the animal moves in. Misting during this period keeps the crew alive and lets you see whether the drainage layer is filling too fast.
How much substrate do you need?
| Enclosure (L × W) | Substrate at 8 cm | Coir needed (4/9 of the mix) | From one 5 kg brick (≈75 L coir) |
|---|---|---|---|
| 45 × 45 cm | ≈ 16 L | ≈ 7 L | 10 enclosures |
| 60 × 45 cm | ≈ 22 L | ≈ 10 L | 7 enclosures |
| 90 × 45 cm | ≈ 32 L | ≈ 14 L | 5 enclosures |
| 120 × 60 cm | ≈ 58 L | ≈ 26 L | 3 enclosures |
A 650 g pet-shop coco brick is about an eighth of a 5 kg brick. If you keep several enclosures, or a larger one, the arithmetic quickly favours the big brick; the unused portion keeps indefinitely once dry (storing coco coir).
Which animals suit a coir-based bioactive set-up
- Crested and gargoyle geckos, day geckos — humid, planted, arboreal; the standard recipe as written.
- Dart frogs, tree frogs, other amphibians — add the optional tree fern fibre or husk chips and keep the drainage layer generous; amphibians are the most sensitive to salts and additives, so washed coir matters most here.
- Royal pythons, boas, tropical colubrids — a coir-heavier, less barky mix holds the 50–70 % humidity that helps them shed cleanly; go deeper for burrowers.
- Tarantulas, millipedes, giant African land snails, stick insects — plain damp coir 5–10 cm deep is the standard bed; snails and millipedes appreciate leaf litter and a source of calcium on top.
- Isopod and springtail cultures — 3 parts coir : 1 part crushed leaf litter with a piece of charcoal or bark to hide under is a workable culture medium.
- Not suitable: bearded dragons, leopard geckos, uromastyx and other arid species, which need a dry substrate. See the species guide, and the tortoise substrate guide for chelonians, where coir is a component rather than the whole bed.
A simpler mix for closed plant terrariums and jars
A sealed glass terrarium with ferns, fittonia and moss has no animals to worry about, so the priorities are drainage and freshness: 2 parts coir : 1 part horticultural grit or perlite : ½ part charcoal, over a 2 cm layer of gravel or clay balls and a thin layer of dried sphagnum to keep the two apart. Keep it barely damp — a closed terrarium recycles its own water — and skip fertiliser entirely, which only feeds algae on the glass. Coir's low nutrient content is an advantage here rather than a limitation.
Common mistakes
- Coir on its own. It compacts and goes anaerobic when permanently wet. Always add bark or chips and leaf litter.
- Skipping the drainage layer. Misting adds up; without somewhere for water to go the substrate becomes a swamp and roots and isopods drown.
- Nursery compost on plant roots. Rinse it off. Fertiliser and perlite do not belong in an animal enclosure.
- Adding the animal on day one. Let the clean-up crew establish for a few weeks first.
- Unwashed coir. Salt residues are the one way coir can harm amphibians and invertebrates. Buy washed, low-EC coir.
Frequently asked questions
Can I use coco coir as a bioactive substrate?
Yes, as the base — around 40–50 % of the mix by volume — with orchid bark, sphagnum, charcoal and leaf litter to keep it open. Used alone it compacts.
Is coco coir the same as plantation soil or coco fibre?
Yes. Reptile brands sell compressed coco coir under names such as plantation soil, coco fibre, coco brick and eco earth. It is the same coconut-husk pith sold to gardeners as coco coir or coco peat.
How deep should bioactive substrate be?
8–10 cm for most planted enclosures, 15 cm or more for burrowing snakes and for plants with big root systems, over a 3–5 cm drainage layer.
Do I need to sterilise coco coir before using it?
No — for a bioactive system you want it alive, not sterile. Coir is a clean, processed product; if you are cautious about hitchhikers, freezing the expanded coir for 48 hours is the usual keeper's precaution. Do not bake it near anything flammable.
How often do I replace bioactive substrate?
Ideally never — that is the point of bioactive. Top up leaf litter as it breaks down, siphon the drainage layer when it fills, and refresh a corner if it goes sour. A conventional (non-bioactive) coir bed is spot-cleaned daily and changed every few weeks to months.
Washed, low-salt coco coir with no added fertiliser or chemicals — 75 litres per 5 kg brick, enough bioactive base for five 90 cm enclosures, free UK delivery.
Shop the 5 kg brick → Coco coir for reptiles & terrariums — species guide