Back to the mud holes...

If you haven't figured it out by now, I'm kind of partial to unusual habitats. I'm sort of obsessed with weird environmental niches that we may have not only overlooked- but perhaps not even considered before.

And perhaps one of the easiest to overlook is the idea of a mud hole.

You heard me.

A mud hole

Yeah, on the surface, this sounds easy and just like a breeding jar or something: Throw peat on the bottom, add your killies...yeah, "mudhole"; whatever..Next. "Fellman, you just described a breeding jar for killies...!"Uh-uh. NOPE. Not gonna let you get away that easy. I'm talking more of a paludarium-type setup, with a strong terrestrial component. Perhaps something more liek the "Urban Igapo" idea I'[ve pushed so much around here. 

You'd probably set it up not unlike a terrarium for reptiles: Somewhat barren, with patches of terrestrial vegetation here and there, and a substrate really consisting of a mix of peat/mud/fine sand. The emphasis more on recreating the overall look  and function than solely the utility of function.

It's not for temporary breeding, either. We're talking "display tank" here!

And it would be shallow water; probably not exceeding 6-8 inches (15.24-20.32cm) in depth. Circulation? Nope.  Filtration? Not likely. You'd be relegated to small water changes with a plastic cup a couple of times a week or more. Even that is not 100% accurate to the biotope, but infinitely more "realistic" than what a lot of us have done before. You'll have dark, acidic water, a small population of fishes, and the challenges of managing bioload in a filter-less "water hole."

"Great, a stagnant tank full of muddy water in my living room. Great fucking idea, Scott. Right on."

Obviously, the starting point for replicating a mud hole in an aquarium is...well...MUD! Er, at least, the stuff you use to create the bottom.

For some time, I've been very intrigued about the terrestrial and other soils that hobbyists who keep "dirted" planted aquariums have utilized for years to facilitate amazing plant growth. However, I'm not talking about them for growing plants- I'm talking about using these materials for the primary substrate in the natural, botanical-focused aquarium, in which plants may or may not play a role.

Now, sure, there are considerations- such as an influx of a lot of nutrient-laden materials into the aquarium (not as important if you're growing plants, of course), and the sheer "messiness" of soils, clays and silt-which have created some consternation among those who use them. Sure, these materials are easily disturbed and can create some rather turbid conditions in the tank as they settle. 

Right from the start, you have a pretty good idea about why this practice isn't exactly taking the aquarium world by storm, is it?


Hell, what could go wrong? 😆

I've heard about concerns over gasses and such being trapped under the soil substrate (likely more of a concern when you're employing a "cap" of sand or other material on top of the soil/silt/clay to retain it) and being released into the tank during maintenance and other activities. Now, in my experiments, I have not experienced this. And I'm not using tremendous depths of sand.

I don't use a "sand cap" on top of my "dirt"- rather, I tend to mix in bits of crushed leaves, botanicals, and twigs, which seems to not only keep the materials together, but enhances the natural, "random" look. I gradually saturate and "flood" these tanks, a sort of analog to what happens in Nature during the periods of inundation in the forests.

I'm sure that I'll get a dozen emails from hobbyists telling me that it's irresponsible snd dangerous to utilize such an approach to substrate in a fish-focused tank, but in almost 7 years of personal experimentation with these types of mixes, I've never had any issues whatsoever- other than the aforementioned cloudiness when the substrate is disturbed. In fact, after a few months, even when the substrate is disturbed in one of these tanks, the cloudiness tends to not occur. Based on my personal experience, I believe that the longer this stuff is down, the more likely it is to STAY down.

Now, does this mean everyone should ditch the time-proven commercial substrate materials and jump head long into creating dirt and silt substrates in their display aquairums?

Of course not.

However, I think it's worth experimenting with.

It's very important to look at our long-held opinions about what aquarium substrates "should" be, and what their role is in the aquarium. We've long offered a variety of materials which we've rather generically called "substrate additives"- stuff you can mix in with conventional sand, soils or use as a primary substrate in experimental systems. Many of you have used our coconut-based coir substrate material, "Fundo Tropical"  or the finer "Substrate Fino" for this purpose over the years as an alternative to peat and such, and it remains a best-seller for I think you're finding interesting uses for this stuff, too.

I think that we should look at substrates in our aquariums as more than just "the bottom" or "a place to put rocks and wood and plants"- but rather, as a dynamic, living, integral component of a balanced closed ecosystem. A place to culture supplemental food organisms, facilitate reproduction of fishes (I'm thinking soil-spawning killies here again), and impact the chemical composition of our water.

It would be great to apply as much emphasis to substrate in this vein as we do to other components of the aquarium. It's about mental shifts; re-thinking the "how's" and "why's" of what we've done for so long.

A "substrate" can be- should be- way more than gravel or plain old sand.

And if we have our say in the matter, it will be!

And of course, if we dip back into Nature for some inspiration- as we should- there is an amazing amount of ideas to take away.. 

Muddy habitats are usually associated with ephemeral habitats, known to ecologists as vernal pools.

Vernal pools are generally found on plains or grasslands, and are typically small bodies of water- often just a few meters wide. The origin of the name, "vernal" refers to  the Spring season. And, this makes a lot of sense, because most of these ephemeral habitats are at their maximum water depth during the Spring!

Vernal pools are typically found in areas comprised of various soil types that contain clays, sediments and silts. They can develop into what geologists call "hydric soils", which  are defined as, “...a soil that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part.”

That's interesting! 

A unique part of the vernal pools is what is an essentially impermeable layer of substrate called "clay pan."  These substrates are hugely important to the formation of these habitats, as the clay soils bind so closely together that they become impermeable to water.  Thus, when it rains, the water percolates until it reaches the "claypan" and just sits there, filling up with decaying plant material, loose soils, and water.

So, yeah- the substrate is of critical importance to the aquatic life forms which reside in these pools! Let's talk killies for a second!  One study of the much-loved African genus Nothobranchius indicated that the soils are "the primary drivers of habitat suitability" for these fish, and that the eggs can only survive the embryonic period and develop in specific soil types containing alkaline clay minerals, known as "smectites", which create the proper soil conditions for this in desiccated pool substrates.

The resulting "mud-rich" substrate in these pools has a low degree of permeability, which enables water to remain in a given vernal pool even after the surrounding water table may have receded! And, of course, a lot of decaying materials, like plant parts and leaf litter is present in the water, which would impact the pH and other characteristics of the aquatic habitat.

Interestingly, it is known by ecologists that the water may stay alkaline despite all of this stuff, because of the buffering capacity of the alkaline clay present in the sediments!

And, to literally "cap it off"- if this impermeable layer were not present, the vernal pools would desiccate too rapidly to permit the critical early phases of embryonic development of the Nothobranchius eggs to occur. Yes, these fishes are tied intimately to their environment.

(Image by Andrew Bogott, used under CC BY-S.A. 4.0)

The fascinating concept of embryonic diapause ( a form of prolonged, yet reversible developmental arrest) is well-known to scientists and lovers of annual killies. The occurrence and length of time of diapause varies from species to species, yet is considered by scientists to be an evolutionary adaptation and ecological trait in various populations of Nothobranchius, tied directly into the characteristics of the ephemeral habitats in which these fish reside!

Diapause assures species survival by enabling the annual life cycle of these fish to be completed, and can even be affected by the presence of adult fishes in the habitat (not a good idea to hatch if potential predators are around, right?)- a fascinating adaptation! Since the embryonic phase of most Nothobranchius is a relatively long period of their lives- and in some species- the longest phase of their life, factors which impact embryonic development are extremely important. 

A temporary pool over a muddy, leaf-strewn substrate can be a fascinating home display, if you ask me. How can we not be tantalized by these natural habitats?

There's a lot going on there!

And we can replicate this habitat quite easily in our aquariums.

If you've been in the hobby long enough, you start noticing how things truly evolve over the years, and how easily we get comfortable doing stuff that, less than a decade before was considered "risky", "non-sustainable", or downright dangerous.

I think so much of it starts with making mental shifts and appreciating the challenges associated with doing stuff slightly different than we have in the past. In other words- simply trying. It seems like there is a certain audacity to doing stuff fundamentally differently than we have in the past; call it what you will- but it's that simple. 

Mindset shifts are beautiful things, because they get us out of our comfort zones and compel us to look at where we were, where we are, how we got there, and where we are going next.

And it often starts with simply playing with mud.

Stay innovative. Stay creative. Stay observant. Stay bold...

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


3 Responses


October 21, 2022

Thank you for your job. You are real pionier!
We definitely shoud observe nature

Scott Fellman
Scott Fellman

October 14, 2020

Hi Leigh,

Thanks for the kind words!

The grass is actually Across, a marginal species that does really great in these kinds of muddy, semi submerged conditions. I’ve also played with various rice varieties, which also do well under these conditions!

Hope this helps



October 13, 2020

Great article Scott, many years ago I tried raising some killies with limited success but I’m interested in trying again with more knowledge available online and after seeing some of your scapes I’m realising it doesn’t have to be something done away from the missus but that I could potentially make a feature of it in the living room without using a lot of space.
My question though, what is the grass you are using in the picture on this article?

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