Accumulating detritus, "mulm", or whatever you call it- moving minds.

Remember from your hobby history (or for some, from your personal experience) those charming aquarium books from the late 1950's/1960's, arguably the hobby's "Golden Age?"

One word I remember seeing in many of these books was "mulm." It was a funny word. A sort of 1950's-60's-style catch-all expression for "stuff" that accumulates at the bottom of an aquarium.

Charming, even.

It was- is- quite appropriate and descriptive!

"Mulm" is similar to the catch-all term of "detritus", which is used in the hobby extensively to describe the solid material that accumulates at the bottom of an aquarium as the end product of biological filtration.

"Mulm", however, is a bit more...

I think mulm is also that "matrix" of stringy algae, biofilms, and fine particles of "stuff" that tends to accumulate here and there in healthy aquariums, What's cool about this stuff is that, not only do you see it in aquariums- you see it extensively in natural ecosystems, such as tropical streams, flooded forest floors, and ponds.

In the case of a botanical-style aquarium, "mulm" is also the broken-down leaves and botanicals. It's a part of what we love to call "substrate enrichment" in our aquariums.  Stuff that physically comprises the bottom of the tank! As botanicals break down- just like in nature, they create a diverse matrix of partially decomposing plant materials, pieces of bark, bits of  algae, and some strings of biofilm.

I mean, I suppose if you want to get all technical and geeky about it, you could refer to a solid definition of "detritus" and work from there:

"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)


Okay, that sounds pretty "official..." I mean, it's one of our most commonly-used aquarium terms...and one which, well, quite frankly, sends shivers down the spine of many aquarium hobbyists, and just flat-out scares the shit out of others!  And judging from that definition, it sounds like something you absolutely want to avoid having in your system at all costs. I mean, "dead organisms" and "fecal material" is not everyone's idea of a good time, ya know?

Yet, when you really think about it, "detritus"  or "mulm", if you want to differentiate-  are an important part of the aquatic ecosystem, providing "fuel" for microorganisms and fungi at the base of the food chain in tropical streams. In fact, in natural blackwater systems, the food inputs into the water are channeled by decomposers, like fungi, which act upon leaves and other organic materials in the water to break them down.

In years past, as well as today, those of us who favore "sterile-looking" aquaria will be horrified to see this stuff accumulating on the bottom, or among the driftwood. Upon discovering it in our tanks, for many hobbyists, it takes nanoseconds to lunge for the siphon hose to get this stuff out ASAP!

In our case, we embrace this stuff for what it is: A rich, diverse, and beneficial part of our microcosm. It provides foraging, "aquatic plant "mulch", supplemental food production, a physical place for fry to shelter, and it's a vital, fascinating part of the natural environment we are working to foster in our tanks.

It is certainly a new way of thinking when we espouse not only accepting the presence of this stuff in our aquaria, but actually encouraging it and rejoicing in its presence! Why? Well, it's not because we are thinking, "Wow, this is an excuse for being lazy and maintaining a dirty-looking aquarium!"


We rejoice- because our little closed microcosms are mimicking exactly what happens in the natural environments that we strive so hard to replicate. Granted, in a closed system, you must pay attention to water quality, but accepting decomposing leaves and botanicals as a dynamic part of a healthy, closed ecosystem is embracing the very processes that we have tried to nurture for many years by removing every drop of the stuff!

Sure, it's a very different aesthetic: Brown water, leaves, stringy algae films, and botanical debris. We may not want to have an entire bottom filled with this stuff...or, maybe we might!

I think that there is a serious distinction between the idea of letting some natural processes play out in the aquarium, and not rushing to remove every gram of detritus from our tanks immediately, and allowing some of it to fuel beneficial biological processes.

I'll admit that, to most modern aquarium hobbyists, the stuff  just doesn't look that nice, and that's at least partially why the recommendation for a good part of the century or so we've kept aquariums has been to siphon it the hell out! And that's good advice from an aesthetic standpoint- and for that matter, from a husbandry standpoint, as well. Well, typically. If it creates good habits and encourages observation and diligence in tank management...On the other hand, why are we so militant about removing the stuff at first sight?

I mean, is this material causing a problem for your fishes? Or does it just look yucky?

How do you know if this stuff is really a "problem" in your aquarium?

Well, first, get back to the basics. Is your "detritus" or "mum" a matrix of uneaten food, or a less-menacing mix of botanical "fines", fish waste, and biofilms? If it's uneaten fish food, well- feed less...right? That's not "detritus accumulation"- it's overfeeding. Straight-up.

Check your water parameters.

Are you seeing surging nitrate or phosphate levels? Excessive growths of algae? Do you have any detectible ammonia or nitrite? Are the fishes healthy, relaxed, eating, and active? If the answer to the first two questions is "no", and the last is "yes"- then perhaps it's time to simply enjoy whats happening in your aquarium! To accept and understand that the aesthetic of a heavily botanical-influenced system is simply different than what we've come to perceive as "acceptable" in the general aquarium sense.

It's not for everyone.

It's not something that we are used to seeing. I totally get this.

And yeah- if you're experiencing issues with your aquariums and the health of your fishes under these circumstances, it's time to review those basics yet again. Think about an aquarium as a sort of "balancing act"- trying to not have too much or too little of anything.

It's a challenge for some of us to achieve that "balance", as it's been known as.

However, the feedback we've been getting from you- our customers- regarding the systems you've set up in this fashion is that they have created an entirely new perception and understanding of a freshwater aquarium. They've enabled us all to try a completely different aesthetic experience, and more important- to understand and encourage processes that occur naturally, which are of great benefit to the fishes we keep- despite long-held beliefs or aesthetic assumptions about what is "good."

Since we've started Tannin, we've heard a lot of stories from hobbyists of successful spawning and rearing of fishes that have proven challenging in the past. We've hear of hobbyists being extremely skeptical and, well- even a bit turned off by what was happening in their water- and then waking up one day and noticing that their fishes have never looked better- never acted more "naturally"- and that visitors to the fish room are fascinated by the "brown tank" that was recently set up...drawn to it.

I've seen this before many times, myself.

I'm not sure why..I don't know if it's simply because these types of tanks are such a radical aesthetic departure from what we're used to, or if it's something more?

Perhaps, we're somehow drawn to their earthy, "organic" vibe?

Perhaps there is something "liberating" about allowing our aquariums to look and work as Nature intends them to; to embrace form and function- rather than to wage war with anything that challenges our ingrained "aesthetic sensibilities?"

I'm not sure.

But I am sure that I'm enjoying my tanks, and so are many of you who have tried this approach. You're having a lot of fun- even with "mulm" in your tanks. Or "detritus"- or...whatever the hell you want to call it.

And fun is what it's all about!

Stay open-minded. Stay curious. Stay studious. Stay bold. Stay diligent. Stay observant...

And Stay Wet.


Scot Fellman

Tannin Aquatics

Scott Fellman
Scott Fellman


6 Responses

Paddy Eason
Paddy Eason

September 06, 2023

Usage of the word ‘mulm’ in English -

Seems to come from German -


March 14, 2023

I have loads of mulm. It appears to be mostly partially decayed bogwood, I’ve had the bogwood several years now and it may be time to retire it. The other components of the mulm appear to be beneficial bacteria colonies, decayed plants, and snail poop (I have way too many mts).

Doesn’t seem to harm the fish but I do worry that it significantly lowers the pH. My tap water is already quite soft and with the mulm and the bogwood it doesn’t take long to use up all the buffering capability (which is almost always 0 in my tank). I vacuum as much of the mulm out as I can every water change but it’s always back by the next change.

I’ll be sad to see the bogwood go, it’s been the hiding place and spawning place of so many fish over the years. It also has several anubias growing on it and is the centrepiece of the tank decor. Maybe I can put it in an emersed set up instead, terrarium or riparium perhaps.

Scott Fellman
Scott Fellman

September 15, 2019

Hi Sarah,

Welcome to the coolest hobby around! I honestly don’t think that “mum”, per se, is really a bad thing. Particularly in a case like yours, where the water parameters are just fine. I think that it’s more problematic when the “mule” consists of excess fish waste and uneaten food, the tell tale signs of poor husbandry! Sounds like you’re doing just fine. What’s “too much mule”, in your case? Likely if it impacts aesthetics..If the water quality is good, it’s hard to pin it on the "mumble, IMHO. Now, the TDS increase could literally be anything, right, as TDS isn’t necessarily a measure of a specific compound. Hard to say what the issue is with the dories. Labored breathing, in the absence of obvious disease symptoms or water quality issues, could be due to lower dissolved oxygen in the water. Perhaps- and I know it’s a stretch- you might want to put some supplemental aeration into the tank…maybe increase oxygenation in this tank could help? Just a thought. And of course, stabilizing any pH swings greater than the typical day/night fluctuations is. always a good idea.

Wish I could offer some better insights!



September 14, 2019

I appreciate this article! I’m a newbie and I’m nervous I have too much mulm. It’s not all food, but just in case I overfeed I added some MTS.

Just curious, when is there too much mulm? My nitrites, ammonia 0ppm and nitrates 0-20ppm, although rarely does it get past 10ppm. I have a 3gal that’s admittedly overstocked, but water parameters are stable. However, my cories recently have been lethargic (but no red gills, labored breathing, or staying at the surface). I think it’s the TDS that’s impacting them. My Fluval Stratum buffers the water, lowering pH. It strips my KH to 0, keeps my pH 6.3-6.8, and keeps my TDS around 200ppm. I’m not sure if I have too much mulm or it’s the higher TDS. When my pH was lower, my TDS was lower, and the cories seemed fine. But I want a more neutral pH and some KH to avoid pH swings.

Since mulm can harbor fry and shrimplets, as well as beneficial bacteria, when should I gravel vac? I’ve been gravel vac’ing almost daily now since I noticed the TDS creep up as I assumed it was due to mulm. If mulm is so beneficial, would a heavily planted tank use up the mulm before it impacted water parameters? I’ve read hydrogen peroxide can neutralize mulm… is that a bad thing? Appreciate any insight you can provide for a noob :)

Scott Fellman
Scott Fellman

May 18, 2019

Ahh- I will have to look back into my Dad’s library of fish books to find exact titles/passages. There is a book called “Fancy Guppies” with Axelrod, Hahnel, and a few other co-others from around 1955 that mentions the term “mule”, and yet another guppy book that my father had on guppies in which several of the well known breeders of the day (Hahnel, Sternke, Alger, etc.) described their tanks and maintenance regimens, and made references to “mulm.” I’ll have to find those passages. Also, reference to “mulm” was found in a few of the Pet Library “Know” series books from that period and the 1960’s ie; “Know Your Goldfish”, Know Your Guppies", etc…Ahh, he books of my childhood (although many years old even then. That term “mulm” is too old to have bene referenced in Baensch’s atlas for sure…I too, would love to know who coined/adapted the term. I agree- not in Innes…I think it came around in that 1950’s-60’s time frame. I’ll have to scan the old library on this end, too! -Fun stuff!


Jerry Smith
Jerry Smith

May 17, 2019

Scot, you mentioned seeing the word “mulm” in 50’s-60’s fish books. I am just starting to see if I can find who coined the word and when they did it. I read it in the Jan-Feb 1986 (vol 1#3) issue of The Aquatic Gardener. There is was used to refer to floating material that settles on plant leaves. I have scanned a few older books in my possession and so far have not found it. I did not see it in Exotic Aquarium Fishes by Wm. T. Innes. I did not see it in The Aquarium Encyclopedia, edited by Gunter Sterba and Dick Mills, published by MIT Press. I didn’t see it in the Baensch Aquarium Atlas vol 1. I have not looked in Vol 2,3 yet. I don’t own any further volumes. Do you have any specific books where you’ve seen it? I’d be interested in seeing them.

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