Depth Above the False Bottom: Burrow Space You Are Not Getting

A 90 x 45 x 45 tank, a 10 cm false bottom, a 3 cm drainage gap and 8 cm of substrate. That is 21 cm of the tank’s height spent before an animal can dig, and 8 cm of actual soil in a tank people describe as “deep”. A 45 cm cube with 15 cm of packed soil gives a burrowing frog nearly twice the usable depth in half the glass.

False bottoms are sold on drainage. They are rarely discussed as a depth tax, and the tax compounds: the taller the tank, the more likely the keeper fills the bottom with structure and runs out of soil.

Start with the animal, not the tank

A useful working rule for committed burrowers is 1.5 to 2 times snout-to-vent length in loose substrate, and 2.5 times for species that build chambers and stay down for weeks. A 7 cm frog therefore wants 12 to 18 cm of usable depth. If your build cannot give that, the honest answer is a smaller false bottom or a different tank, not a shallow layer and a hide.

Species differ enormously. Some frogs dig a scrape and sit in it; some salamanders move horizontally through leaf litter rather than down. Getting the figure wrong in either direction is a welfare issue, so check the requirement for the species you keep rather than applying a rule of thumb from a forum.

The depth tax, in numbers

Usable means from the surface to the point where the substrate meets the screen, measured with a ruler after the layer has settled for a week. A 14 cm layer compacted by misting and by the animal’s own digging will be 11 to 12 cm in a month. Measure after settling, not on the day you built it.

The failure worth avoiding

A keeper built a 90 cm for a group of tomato frogs with a 10 cm false bottom, then added 15 cm of loose coco and gel mix. It looked right for three weeks. Then the layer settled, the frogs dug down to the screen, and the tank began to hold water at the bottom because the substrate had slumped and sealed the drainage. The animals were fine; the tank was rebuilt at a cost of a weekend and a lot of wasted material.

The fix was not more substrate. It was a lower false bottom, 5 cm instead of 10, and a mix that packs. A burrow has to hold its shape. Loose, fluffy substrate at 18 cm is worse than packed substrate at 12, because the animal digs and the tunnel closes behind it, which is exactly what a chamber-builder cannot use.

Packing, and how to test it

• Mix at 3 parts coco to 1 part hydrated gel by volume, plus 10 percent by volume of fine bark for structure.
• Add in 4 cm lifts and press each lift with a flat hand until it stops compressing.
• After the last lift, push a finger in to the second knuckle. The hole should stay open for a minute.
• If the hole closes, the mix is too loose; add a handful of fibre and repack.
• Keep one corner deliberately loose and dry for surface species that do not burrow.

A packed layer also holds moisture more evenly, which is the part that matters for substrate depth and humidity balance. A loose layer dries from the top 3 cm while the bottom stays wet, and the animal sits in the wet part.

A false bottom you can live with

For a burrowing species, shallow is better: 4 to 5 cm of egg crate with a drain tap, a weed barrier across the top, and the depth budget going into soil. Keep the drain accessible; if you cannot reach it, you will not use it. Slope the false bottom 1 cm towards the tap so water collects where you can remove it.

Then measure. Write the settled depth on a label and stick it to the tank frame. When the number drops below the animal’s requirement, it is time to rebuild, not to add another inch on top and hope.

Take a ruler to your tallest tank tonight and measure from the surface to the screen at the front glass. Most keepers find 3 to 5 cm less than they assumed.