November 3, 2026 · Matt J

Drainage Layer Guide: Clay Balls vs False Bottom in a Bioactive Terrarium

Quick answer: Tropical bioactive enclosures should have a drainage layer. Use 3–5 cm of clay balls covered with a mesh barrier, then 8–10 cm of soil. False bottoms (egg-crate platforms) suit very wet builds and paludariums. Desert and semi-arid builds usually skip the drainage layer and use a deeper soil instead.

What a drainage layer does

In a tropical vivarium, you add water every day through misting. Without somewhere for excess water to go, it collects in the bottom of the soil, oxygen is driven out, and the substrate turns anaerobic. That means a sour smell, root rot and dying clean-up crew. A drainage layer:

gives excess water a reservoir separate from the soil;
keeps roots in moist but aerated soil;
maintains humidity as water slowly evaporates back up;
gives you a visual check of water levels through the glass.

Option 1: Clay balls (LECA)

Expanded clay balls are lightweight, porous and inert. They're the most popular drainage choice for bioactive builds.

Pros: light, easy to pour in, holds some moisture to boost humidity, reusable, works in any shape of enclosure.
Cons: takes up some height, can float if the enclosure floods.
Depth: 3–5 cm for most terrariums, up to 7–8 cm for tall vivariums with heavy misting.

Rinse clay bio balls before use to remove dust. Our clay balls come from Jurassic Reptile Products, a Canadian bioactive brand.

Option 2: False bottom

A false bottom is a raised platform, usually egg-crate (plastic lighting diffuser) on small PVC legs, covered with mesh. It creates an empty water reservoir underneath.

Pros: holds a lot of water, lightweight, ideal for very wet setups, can house a pump for waterfalls.
Cons: more work to build, needs a tight seal at the edges so soil doesn't fall through.
Best for: paludariums, heavily misted dart frog vivariums and enclosures with water features. See paludarium guide.

Option 3: No drainage layer

Some builds don't need one:

Desert and semi-arid builds (leopard geckos, bearded dragons, uromastyx): you water rarely, so deep soil like Jurassic ABG Semi-Arid Blend handles it. See leopard gecko build.
Burrowing tarantulas and scorpions: deep substrate matters more than drainage. Use Jurassic ABG Tarantula Mix.
Small closed jars: many builders use a thin layer of clay balls or none, and water sparingly. See closed jar guide.

Comparison table

	Clay balls	False bottom	No drainage
Best for	Most tropical builds	Very wet builds, paludariums	Desert, invert and jar builds
Effort	Low	Medium-high	None
Water capacity	Medium	High	Low
Weight	Light	Very light	N/A
Typical depth	3–5 cm	3–8 cm	N/A

The barrier layer

A barrier keeps soil from washing into the drainage layer and clogging it. Options:

Fibreglass window screen: cheap, durable, easy to cut. Most common.
Purpose-made substrate barrier mesh from pet stores.
A layer of dry sphagnum moss: natural and easy, though it eventually breaks down into the soil.

Avoid landscape fabric treated with chemicals, and metal mesh that can corrode.

How much soil goes on top?

For tropical builds, 8–10 cm of bioactive soil over the drainage is a good target. That's deep enough for roots and the clean-up crew, with a gradient from moist at the bottom to drier on top. Canadian-made Jurassic ABG Terrarium Mix from Jurassic Reptile Products stays airy at this depth. Use our substrate calculator to work out exact litres of both drainage and soil. See also how much substrate do I need?

Managing water in the drainage layer

Check the layer through the glass weekly.
Water should never rise above the barrier into the soil.
If it does, remove excess water with a turkey baster, a syringe and tube, or a siphon. Some builders install a vertical PVC tube in a back corner reaching down into the drainage layer, which makes removal easy.
Reduce misting or improve ventilation if the layer keeps filling up quickly.