What goes on down there?

If you've been following us for any appreciable length of time here at Tannin Aquatics, you've probably surmised that we're obsessed with creating what we call "botanical substrates"- aggregations of various botanicals, ranging from leaves to twigs to seed pods- to serve as a functional and aesthetic feature in our aquariums.

It's a more or less direct mimic of Nature, where leaves and other materials accumulate in streams, ponds, grasslands, and flooded forest areas.

We've found over the years of playing with botanical materials that substrates can be really dynamic places, and benefit from the addition of leaves and other materials. For many years, substrates in aquarium were really just sands and gravels. With the popularity of planted aquariums, new materials, like calls and other additives, entered into the fray. With the botanical-style aquarium starting to gain in popularity, now you're seeing larger materials added on and in the substrate. 

Oh, no, he's talking about that "functional aesthetic shit again!"

Yes. Yes I am. 😎

Let's think about the way I look at the substrate of an aquarium.

When you're operating in our "arena", in a strictly aesthetic sense, the bottom itself becomes a big part of the aesthetic focus of the aquarium. And of course, I see the bottom of the aquarium as more than just sand or whatever. Rather, it's a important component of the aquarium habitat, with the botanicals placed upon the substrate- or, in some cases, becoming the substrate!

These materials form an attractive, texturally varied "microscape" of their own, creating color and interest. In fact, I dare say that one of the next frontiers in our niche would be an aquarium which is just substrate materials, without any "vertical relief" provide by wood or rocks.

We've tried this before, and it worked out pretty well.

FIrst off, what are the potential benefits of constructing a bottom or substrate consisting almost entirely of botanical materials?

Well, first off, such substrates become not only physical places for fishes to hide and forage among- they become an integral part of the entire closed aquarium ecosystem itself, helping influence water parameters, foster growth of fungi and microorganisms, and just maybe- some form of nutrient export/denitrification (although that last part is still a bit speculative).

I realize that experimenting with these unusual substrates requires not only a sense of adventure, a direction, and some discipline- but a willingness to accept and deal with an entirely different aesthetic than what we know and love. And this also includes pushing into areas and ideas which might make us uncomfortable, not just for the way they look, but for what we are told might be possible risks.

Sure, I said risks.

One of the things that many hobbyists ponder when we contemplate creating deep, botanical-heavy substrates, consisting of leaves, sand, and other botanical materials is the buildup of hydrogen sulfide, CO2, and other undesirable compounds within the substrate.

Well, it certainly does make sense that if you have a large amount of decomposing material in an aquarium, that some of these compounds are going to accumulate in heavily-"active" substrates. Now, the big "bogeyman" that we all seem to zero in on in our "sum of all fears" scenarios is hydrogen sulfide, which results from bacterial breakdown of organic matter in the total absence of oxygen.

Let's think about this for just a second.

In a substrate bed consisting entirely of botanical materials, with materials placed on the substrate, or loosely mixed into the top layers, will it all "pack down" enough to the point where there is a complete lack of oxygen and we develop a significant amount of this reviled compound in our tanks? I think that we're more likely to see some oxygen in this layer of materials, and I can't help but speculate- and it IS just speculation- that actual de-nitirifcation (nitrate reduction), which lowers nitrates while producing free nitrogen, might actually be able to occur in a "deep botanical" bed.

And it's certainly possible to have denitrification without dangerous hydrogen sulfide levels. As long as even very small amounts of oxygen and nitrates can penetrate into the substrate, this will not become an issue for most systems. I have yet to see a botanical-style aquarium where the material has become so "compacted" as to appear to have no circulation whatsoever within the botanical layer.

Now, sure, I'm not a scientist, and I base this on close visual inspection of numerous aquariums, and the basic chemical tests I've run on my systems under a variety of circumstances. As one who has made it a point to keep all of my botanical-style aquariums in operation for very extended time frames, I think this is significant. The "bad" side effects we're talking about should manifest over these longer time frames...and they just haven't.

And then there's the question of nitrate. 

Although not the terror that ammonia and nitrite are known to be, nitrate is sort of a biological "yardstick" of overall water quality. Now, as nitrate accumulates, many fish will eventually suffer some health issues. Ideally, we strive to keep our nitrate levels no higher than 5-10ppm in our aquariums.  As a reef aquarist, I've always been of the "...keep it as close to zero as possible." mindset, but that is not always the most realistic or achievable target in a heavily-botanical-laden aquarium.

You have a bit more "wiggle room" with nitrate, IMHO. Now, when you start creeping towards 50ppm, you're getting closer towards a number that should alert you. It's not a big "stretch" from 50ppm to 75ppm and higher...

And then you get towards the range where health issues could manifest themselves in your fishes. Now, many fishes will not show any symptoms of nitrate poisoning until the nitrate level reaches 100 ppm or more. However,  studies have shown that long-term exposure to concentrations of nitrate stresses fishes, making them more susceptible to disease, affecting their growth rates, and inhibiting spawning in many species. 

At those really high nitrate levels, fishes will become noticeably lethargic, and may have other health issues that are obvious upon visual inspection, such as open sores or reddish patches on their skin. And then, you'd have those "mysterious deaths" and the sudden death (essentially from shock) of newly-added fishes to the aquarium, because they're not acclimated to the higher nitrate concentrations.

Okay, that's scary stuff. However, high nitrate concentrations are not only manageable- they're something that's completely avoidable in our aquairums.

Quite honestly, even in the most heavily-botanical-laden systems I've played with, I have personally never seen a higher nitrate reading than around 5ppm. That's no bullshit, and I'm not holding myself up as a shining example of aquaristic perfection. I simply attribute this to common sense stuff: Good quality source water (RO/DI), careful stocking, feeding, good circulation, and consistent basic aquarium husbandry practices (water changes, filter maintenance, etc.).

Now, that's just me.

I'm no scientist, certainly not a chemist, but I have a basic understanding of maintaining a healthy nitrogen cycle in the aquarium. And I am habitual-perhaps even obsessive- about consistent maintenance. Water exchanges are not a "when I get around to it" thing in my aquarium management "playbook"- they're "baked in" to my practice. 

They should be a standard practice for every aquarist who plays with botanical-style aquariums- period.

So yeah, although nitrate accumulation is a potential concern in botanical-style aquariums, it need not be an ominous cloud hanging over our success. In my opinion, the far more problematic issues with botanicals and water quality are related to lapses in our own good judgment and to misguided practices.

Experience with our customer base tends to confirm this, too.

The very few issues that we've seen with people "pushing it too far" in terms of botanical applications, were caused by rapid influxes of large quantities of botanical materials to existing, stable aquariums, which, I believe, overwhelmed the resident bacterial population and might have resulted in rapid oxygen depletion and a corresponding increase in CO2. The result was fishes hanging at the surface in an attempt to get oxygen. The good news was that almost every situation like this I heard of was remedied in a relatively short period of time by adding additional aeration into the tank, a series of water exchanges, and/or removing some of the materials.

Generally, loss of life was minimal ("minimal"-that's an awful term, though) or nonexistent as a result of these measures. Regardless, it's really important to be careful. Fish can die if we push it too hard.  It's not just "Boil, dump...Instant Amazon..."  Measured implementation and experimentation is required when using botanicals. We're often adding biological materials to established aquariums, which might not be able to handle large, fast influxes.

There's obviously some "upper limit" of how much botanical material we can add to a given established  system in a brief period of time, and it's especially more profound in newly-established aquariums with "immature" nutrient export mechanisms in place.

What kinds of things can we do to prevent problems like this?

Well, for one thing, we can add botanical materials gradually, at a slow, steady pace. This will give our bacteria population a chance to catch up with the influx of materials being added. Also, it will slow down the pace of any pH fluctuations (assuming we are utilizing stuff that can lower the pH in our tanks) so that the fishes can adjust to them.

It's common sense "best practice" for us.

Another thing would be to employ good circulation within your system, which not only results in greater oxygenation and "mixing" of water "strata" - it physically suspends fine particulates in your system as well, making it easier for mechanical filtration to remove (of course, that assumes you don't like the look of "stuff" in the water, as some of us do!).

Any debris stirred up can easily be removed mechanically by filtration, as mentioned above. Of course, you don't have to go crazy siphoning the hell out of your sand every week, essentially decimating populations of beneficial microscopic infauna -or interfering with their function- in the process.

Now, I think one of the most "liberating" things we've seen in the blackwater, botanical-style aquarium niche is our acknowledgment that you can and should utilize the substrate itself to become a functional mechanism for its inhabitants.

It's certainly no stretch to call our use of botanicals as a form of "active substrate", much like the use of clays, mineral additives, soils, etc. in planted aquariums. Although our emphasis is on creating specific water conditions, fostering the growth of microorganisms and fungi, as well as creating unique aesthetics, versus the "more traditional" substrate materials fostering conditions specifically for plant growth.

Once your substrate is in place, Nature takes over and the materials develop that lovely "patina" of biofilms and microbial growth, and start breaking down. Some may be moved about by the grazing activities of resident fishes, or otherwise slowly redistributed around the aquarium.

A literal "active substrate", indeed! Yet, something that is fascinating and beautiful for those who give the idea a shot!

Once again, I encourage you to study the natural environment, particularly niche habitats or areas of the streams, rivers, and lakes- and draw inspiration from the functionality of these zones. The aesthetic component will come together virtually by itself. And accepting the varied, diverse, not-quite-so-pristine look of the "real thing" will give you a greater appreciation for the wonders of nature, and unlock new creative possibilities.

It's beyond liberating...In fact, it's something that has really spurred me to re-evaluate the way I interpret botanical-style aquariums. To go beyond the aesthetics and embrace the possible functions- food production, biological filtration- biodiversity- is a fascinating journey.

Over many decades of aquarium-keeping, I have learned to not be afraid of one of the tangential benefits of these types of substrates:


Yeah, detritus.

That thing we all collectively freak out about.

The definition, as accepted in the aquarium hobby is kind of sketchy in this regard; not flattering at the very least:

"detritus is dead particulate organic matter. It typically includes the bodies or fragments of dead organisms, as well as fecal material. Detritus is typically colonized by communities of microorganisms which act to decompose or remineralize the material." (Source: The Aquarium Wiki)

Everyone thinks that it is so bad.

I'm not buying it.

Why is this necessarily a "bad" thing?

I mean, even in the above the definition, there is the part about being "colonized by communities of microorganisms which act to decompose or remineralize..."

It's being processed. Utilized. What do these microorganisms do? They eat it...They render it inert. And in the process, they contribute to the biological diversity and arguably even the stability of the system. Some of them are utilized as food by other creatures. Important in a closed system, I should think!

Yeah, this is really important. It's part of the biological "operating system" of our aquariums. And it's a direct product of much of the materials we keep on the bottom of our aquariums.

It's not all bad, right?

I think we should embrace this. Especially in a botanical-style aquarium, which essentially "runs" on the decomposition of materials.

In the flooded forest floors we find in Nature, the leaf litter "community" of fishes, insects, fungi, and microorganisms is really important to the overall tropical environment, as it assimilates terrestrial material into the blackwater aquatic system, and acts to reduce the loss of nutrients to the forest which would inevitably occur if all the material which fell into the streams was washed downstream!

Stuff is being used by a myriad of life forms.

We've talked about this forever, and I think it has much merit to consider.

Is there a lesson from Nature here that we can incorporate into our aquarium work?

I think so!

Okay, detritus as we see it may not be the most attractive thing to look at in our tanks. I'll give you that. It literally looks like a pile of shit! However, what we are talking about allowing to accumulate isn't fish poop and uneaten food. It's broken-down botanical-materials. 

That's a hugely important distinction.

As we talk about so much around here- just because something looks a certain way doesn't mean that it alwaysa bad thing, right? What does it mean? Take into consideration why we add botanicals to our tanks in the first place. Now, you don't have to have huge piles of the stuff littering your sandy substrate.

However, you could have some accumulating here and there among the botanicals and leaves, where it may not offend your aesthetic senses, and still contribute to the overall aquatic ecosystem you've created.

Think about what the nitrogen cycle is and does, and think about the impact of inputs and exports into and out of our closed systems. 

Ponder the potential benefits of allowing some of this stuff to remain.

Think about the organisms which feed upon it, their impact on the water quality, and on the organisms which fed on them. Then, think about the fishes and how they utilize not only the material itself, but the organisms which consume it.

Consider its role in the overall ecosystem...

Is detritus just a nutrient trap?

Or is it a place for fishes to forage among?

A place for larval fishes to seek refuge and sustenance in? Kind of like they do in Nature, and have done so for eons?

Yes, I know, we're talking about a closed ecosystem here, which doesn't have all of the millions of minute inputs and exports and nuances that Nature does, but structurally and functionally, we have some of them at the highest levels (ie; water going in and coming out, food sources being added, stuff being exported, etc.).

I think we really need to think about our systems- particularly in the blackwater/botanical-style aquarium world- as little microcosms which replicate- at least on some level, some of the process which occur in nature to create a specialized but highly productive and successful- not to mention, dynamic- ecology. 

And not all of these processes have appealing visuals. I believe that we as hobbyists need to separate aesthetics from the overall functional benefits of the various life forms and processes which appear in and guide our aquairums' ecological systems.

There is so much more to this stuff than to simply buy in unflinchingly to overly-generalized statements like, "detritus is bad."

I think that the idea of an "enriched substrate" and the accompanying products of the resident biotic will become an integral part of the overall ecosystems that we create. Considering the substrate as both an aesthetic AND functional component- even in "non-planted" aquariums, opens up a whole new area of aquarium "exploration."

I envision that the future of mainstream aquarium practice may include creating such a substrate as simply part of "what we do." Adding a mix of botanical materials, live bacterial and small organism cultures, and even some "detritus" from healthy aquatic systems may become how we establish systems.

Think on that for a bit. Think about what goes on down there in the substrate...And apply lessons and observations from Nature into your botanical-style aquarium "practice."

It's not some amazing "revolution"- it's simply an evolution of practices that we've been playing with peripherally for decades in the hobby. It's a way of looking at what's already working and trying to figure out the "whys" as we go. 

Let's keep going.

Stay brave. Stay intrigued. Stay observant. Stay thoughtful. Stay diligent...

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


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