No Soil Bioactive Terrarium: Why I Switched to Foam

By Allen Booth and Derick Michael

Allen has 30+ years in the exotics industry. Derick runs MLE's dart frog breeding program in New Jersey.

After years of building terrariums, I always used some kind of soil mix. Usually ABG, sometimes others. I've bought premade mixes and I've made my own, and I liked some better than others. But they all had the same job, and they all came with the same headaches.

A few years ago a lot of the pros I talk to told me they'd switched to foam. No soil at all. I did my research, online and by talking to other breeders, and I gave it a try. I love it. I haven't gone back, and I don't plan to.

This is how I build a no-soil bioactive terrarium, and why I think it's worth switching.

First, let me be fair to soil

ABG and mixes like it are genuinely good. They hold plants well, they buffer humidity, they look natural, and they host the microfauna that make a bioactive setup work. If you love your soil setup, I'm not here to tell you it's wrong. It isn't.

But soil comes with problems that no-soil simply doesn't have, and once I stopped dealing with them, I realized how much I'd been putting up with.

The drainage problem

With soil, the whole thing lives or dies on drainage. Dart frogs want high humidity, but they don't want to sit in soggy ground. If your drainage isn't good and water starts sitting in the substrate, you get anaerobic conditions, and you'll know fast because of the smell. That sour, swampy odor is the sign that things are going wrong under the surface, and mold usually isn't far behind. I've had it happen, and it's not something you want to fix with frogs already living in the tank.

The replacement problem

Even when soil is working, it doesn't last forever. I always found myself replacing it every couple of years, sometimes sooner. That means tearing down a working tank, which nobody enjoys.

Here's the number that sold me on no-soil: I've had terrariums running with no soil for four years now, and I haven't had to replace a thing. I don't actually know how long they'll last, because I tend to rebuild my tanks before four years anyway just because I like to upgrade. But I'm confident they'll go well beyond four years. And in four years of no-soil builds, I have not had mold or a bad smell a single time.

The biggest practical win: you can get under it

This is the one that surprised me most. With a no-soil build, if you're not drilling drainage holes and running hoses, and the water level gets too high, you can just lift up a corner of the foam at the front and siphon the water out. Even with the frogs still in the tank.

No draining system to plumb, no holes to drill, no hoses to hide. If you need to get under the build for any reason, you lift it. That alone is worth the switch for me.

How I build it

I'm not going to cover backgrounds in depth here (that's its own article), but quickly: I do aquarium-safe silicone first, let it dry, then black spray foam. I use Sprayman foam with a gun rather than the little cans with the straw. If you do a lot of builds, the gun system is so much easier, and one can covers about three terrariums depending on size, so it goes much further. I lay cork into the foam, then apply aquarium-safe black silicone heavily and press coco fiber mixed with sphagnum moss into it. That's the background.

The drainage layer

Now the no-soil part. For the bottom, I use egg crate. I don't put the sponge directly on the bottom, because you don't want water sitting in the sponge itself. That's the thing that causes bacteria and smell. You want the water to drain through to the egg crate below, where it can sit without touching the sponge.

I cut the egg crate to fit with garden shears. It's the easy way to get through it cleanly.

You can run egg crate across the whole bottom and just set the sponge on top of it. But I don't like how that looks from the front. So here's what I actually do: I stop the egg crate a little short of the front and leave a small gap. Then I cut small pieces of sponge and stand them up to make a little wall. In front of that wall, I place clay balls (LECA). I think it gives a much cleaner look through the front glass, and it lets me see the water level so I know when I need to siphon.

Yes, that small piece of sponge in the front is technically sitting where water can reach it, which is the thing we're trying to avoid. But it's a tiny amount and I've never had an issue with it. If you want to be one hundred percent sure, you can cut down egg crate and use that as the front wall instead. Clay balls as a full false bottom work too.

The sponge layer

The main layer is sponge, and the type matters. I use 35 to 40 PPI. Good drainage, but not so open that it holds nothing. The sponge comes at that PPI, you're not making it, you're just cutting it to fit, because it comes oversized.

Don't go thick. One inch is plenty. You can go thicker, but there's no reason to, and you lose real estate in the tank for no benefit.

One tip that makes a real difference: use a fresh blade when you cut the sponge. A sharp blade goes through cleanly. A dull one drags and tears and makes a mess of the edge. New blade, every time.

The top layer

I know what you're thinking. Now I've got a sheet of foam as my top layer, and that's not a great look. But we don't leave it that way.

I put down a lot of leaf litter, which is exactly how these frogs live in the wild anyway, and I mix in sphagnum moss to help hold humidity. Everywhere I have a plant, I surround it with sphagnum too. Once the leaf litter and moss are down, you don't see the foam at all. It looks like a forest floor, because functionally that's what it is.

That's it. No soil, and an easy tank to build.

But how do plants grow without soil?

This is the question everyone asks, and the answer is simpler than you'd think: a lot of plants don't need soil at all. I just pack sphagnum moss around the roots and they're good to go. The moss holds moisture at the root zone, the roots take hold, and the plant grows in exactly like it would in soil, without any of the drainage or replacement problems.

The key is choosing plants that work this way. Not everything does, but plenty of the best vivarium plants are happy rooted in moss or growing epiphytically onto the background and wood. That's why we put together no-soil plant packages: plants we've selected specifically because they thrive without soil, so you don't have to guess. You can see the no-soil plant packages here.

Where soil still makes sense

I'm not going to pretend no-soil is right for every single build. If you're doing a heavily planted display tank where you want deep-rooted plants growing across the entire floor, soil still has advantages. And if you've got a soil setup dialed in and it's working, there's no urgent reason to tear it out.

But for how I keep and breed dart frogs, no-soil has been all upside: no replacing substrate, no drainage anxiety, no mold, no smell, and I can lift the whole thing to get underneath whenever I need to. Four years in, I'm not going back.