Don't fear the fungi. Don't bash the biofilms...Celebrate the slime!

As everyone knows, when you put stuff in water, one of four things seems to happen:

1) Nothing.

2) It starts to break down and decompose.

3) It gets covered in a gooey slime of algae, fungal growth, and "biofilm."

4) Both 2 and 3

 Now, it's pretty much a "given" that any botanicals or leaves that you drop into your aquarium will, over time, break down. And typically, before they break down, they'll "recruit" (a fancy word for "acquire') a coating of some rather unsightly-looking growth. Well, "unsightly" to those who have not been initiated into our little world of decomposition, biofilms, tinted water, etc., and maintain that an aquarium is a pristine-looking place without a speck of anything deemed "aesthetically unattractive" by the masses! 

So, with that little explanatory passage out of the way, let's take a closer look at the stuff that you'll see covering the leaves and pods and wood that you place into your aquarium.


First off, our "friend", biofilm. This much-maligned stuff is something those of us who play with leaves and botanicals know all too well. It's something we see in our aquariums, as well as in the wild aquatic habitats around the world.Biofilms form when bacteria adhere to surfaces in some form of watery environment and begin to excrete a slimy, gluelike substance, consisting of sugars and other substances, that can stick to all kinds of materials, such as- well- in our case, botanicals.



It starts with a few bacteria, taking advantage of the abundant and comfy surface area that leaves, seed pods, and even driftwood offer. The "early adapters" put out the "welcome mat" for other bacteria by providing more diverse adhesion sites, such as a matrix of sugars that holds the biofilm together. Since some bacteria species are incapable of attaching to a surface on their own, they often anchor themselves to the matrix or directly to their friends who arrived at the party first.


And we could go on and on all day telling you that this is a completely natural occurrence; bacteria and other microorganisms taking advantage of a perfect substrate upon which to grow and reproduce, just like in the wild. Freshly added botanicals offer a "mother load"of organic material for these biofilms to propagate, and that's occasionally what happens - just like in nature.  

They are not only typically harmless in aquariums, they are utilized as a supplemental food source by a huge variety of fishes and shrimps in both nature and the aquarium. They are a rich source of sugars and other nutrients, and could prove to be an interesting addition to a "nursery tank" for raising fry if kept in control. Like, add a bunch of leaves and botanicals, let them do their thing, and allow your fry to graze on them! Don''t believe me? Ask almost any shrimp keeper-they'll "sing the praises" of biofilm for the "grazing" aspect!

Is there a "darkside" to biofilms? Of course.

Like anything else, too much of a "good thing" can cause problems in rare instances. Frightening, "aquarium armageddon scenarios" could play out. For example, in an extremely overcrowded aquarium (or a very small one) with marginal husbandry and filtration, with a huge amount of biofilm (relative to tank volume) caused by an equally huge influx of freshly-added botanicals, there is always the possibility that bacteria within the biofilms can multiply extremely rapidly, reducing the level of oxygen in the rest of the aquarium, which could lead to a dramatic reduction of CO2 being released out of the water.

This, in turn, could lead to CO2 levels rising quickly and sharply, potentially causing asphyxiation to the animals in the tank- including the lovable nitrifying bacteria that support it. Now, that's the ultimate "doomsday scenario", extremely rare, yet brought on by a "collaboration" between naturally-occurring biofilm and the impatient aquarist who fails to heed our words and go slowly when adding botanicals to an established aquarium! 

Hey, what about the fungi?

Now, like so many things that we do in the aquarium world, we tend to over-generalize and label any hairy, slimy-looking growth in our tanks as "biofilm." Good as a catch-all phrase, but any trained biologist would likely want to "slap us upside the head" for this sinful over-generalization! I mean, you'll actually see the term "fungal biofilms" in scholarly articles, so perhaps they'll lighten up and cut us a little slack?

Fungi are living organisms that are distantly related to plants, and more closely related to animals, but rather different from either of those groups. Fungi essentially consist of molds, yeasts and mushrooms. Yeasts are single-celled organisms (like bacteria) while molds are long branching thread-like filaments (called "hyphae") that form visible colonies. 

They are found in just about any habitat, yet most live on the land, mainly in soil or on plant material, rather than in water. However, a number of different groups of fungi are found in water, including many Mastigomycotina ("zoosporic fungi"), some Zygomycotina, Ascomycotina, Deuteromycotina, yeasts, and several Basidiomycotina species.

Try googling that stuff for fun some time! 🤓

There are more than 600 species of freshwater fungi! Some may inhabit aquatic habitats for their entire lives, while others may be essentially "amphibious", with part of their life cycle spent in, adapted to and dispersed under water, and another part of their life cycle spent dispersed into the air.  Most aquatic fungi have a sort of stringy `tail' which both helps them stay put and capture food. In addition, the head of the fungal structure is surrounded by a gel-like sticky mucus "sheath" which may aid in adhesion to substrates.

Fungi reproduce by releasing tiny spores that then germinate on new and hospitable surfaces (ie, pretty much anywhere they damn well please!). These aquatic fungi are involved in the decay of wood and leafy material. And of course, when you submerge terrestrial materials in water, growths of fungi tend to arise. Anyone who's ever "cured" a piece of aquatic wood for your aquarium can attest to this!


Aquascaping forums online are filled with horrific descriptions and photos of expensive pieces of wood covered in nasty-looking growth shortly after being immersed, with frantic pleas for help from the hobbyist who set the wood into his/her 'scape only weeks before, with visions of a pristine-looking underwater paradise like you see all the cool kids's both sad to see and a bit amusing. (Yes, I'm not always that nice, lol) 

Fungi tend to colonize wood because it offers them a lot of surface area to thrive and live out their life cycle. And cellulose, hemicellulose, and lignin, the major components of wood and botanical materials, are degraded by fungi which posses enzymes that can digest these materials! Fungi are regarded by biologists to be the dominant organisms associated with decaying leaves in streams, so this gives you some idea as to why we see them in our aquariums, right? 

Aquatic fungi have been shown to produce a rich array of enzymes able to degrade the major leaf polysaccharides...Interestingly, as leaves decay, their nitrogen content becomes a higher percentage of their biomass, and biologists assume that a higher nitrogen content in leaves and botanicals during this time indicates higher fungal activity and therefore, the decay rates of leaves and the fungal populations are somehow correlated.


And of course, fishes and invertebrates which live amongst and feed directly upon the fungi and decomposing leaves and botanicals contribute to the breakdown of these materials as well! Aquatic fungi can break down the leaf matrix and make the energy available to feeding animals in these habitats. And look at this little gem I found in my research:

"There is evidence that detritivores selectively feed on conditioned leaves, i.e. those previously colonized by fungi (Suberkropp, 1992; Graca, 1993). Fungi can alter the food quality and palatability of leaf detritus, aecting shredder growth rates. Animals that feed on a diet rich in fungi have higher growth rates and fecundity than those fed on poorly colonized leaves. Some shredders prefer to feed on leaves that are colonized by fungi, whereas others consume fungal mycelium selectively..."

"Conditioned" leaves, in this context, are those which have been previously colonized by fungi! They make the energy within the leaves and botanicals more available to higher organisms like fishes and invertebrates! What a deal! 


So yeah, a lot of information to unpack there...most of it pretty damn good! The reality to us as "armchair biologists" is that the presence of these (aesthetically unpleasant-looking) organisms in our aquariums is not only a sign that our closed microcosms are functioning well, but that they are, in their own way, providing for the well- being of the inhabitants! 

The "mental stretches" that we ask you to make to accept these organisms and their appearance really requires us to look at the wild habitats from which our fishes come, and reconcile that with our century old idealization of what nature (and therefore our "natural" aquariums) actually look like.

It's not an easy stretch for most. 

Sure, it's not everyone's idea of "attractive", and you'd freak out snobby contest judges with a tank full of biofilms and fungi, but to most of us, we should take great delight in knowing that we are providing our fishes with an extremely natural component of their ecosystem, the benefits of which have never really been studied in the aquarium, because we're too busy looking for ways to remove the stuff instead of watching our fishes feed on it!


And the reality is that you can scrape and siphon the stuff off of your fancy driftwood...but it will typically come back until the food source ( the surface compounds of the wood) is exhausted or until the biofilm and fungi themselves are consumed by your fishes or shrimp. This, of course, is why most hobbyists tend to "cure" their wood in a separate container over the course of a few weeks or more, rather than "in situ" in the display. Although some of the aforementioned benefits created by this process, in my opinion, might cause a few brave souls to re-think this process! 

Still other hobbyists blast away at these organisms with toothbrushes and syringes of peroxide and accelerate the process to some extent. This article is more about understanding and accepting/appreciating the benefits of these organisms rather than the techniques to remove them, so you'll have to deep dive into some other forum or discussion group to garner the approach to that sort of thing. 😆


The acceptance of these organisms and appreciation for the potential benefits they can provide for our fishes and the aquatic ecosystem itself is one of the real features and potential game-changers that the botanical-style aquariums that we love so much can offer. It's a lot different than what we've all become accustomed to. However, the upside to embracing these gooey-looking growths seems to be far, far greater than the challenges of re-training ourselves to appreciate their appearance.

Be bold...Learn to love your Mastigomycotina- because your fishes probably already do! 

Stay open-minded. Stay curious. Stay experimental. Stay enthralled. Stay resolute... 

And Stay Wet.


Scott Fellman 

Tannin Aquatics 








Scott Fellman
Scott Fellman


3 Responses


April 22, 2021

Como se llamo esto i am from SPAIN

Scott Fellman
Scott Fellman

July 01, 2019

Can’t be 100% certain without seeing it, but it sounds like what is called “Black Beard Algae” in the hobby. This is usually caused by excesses of nutrients and/or low levels of CO2 in tanks with plants (ie; they outcompete the plants for available nutrients in lower CO2 environments). It may be manually removed and there are other practices around to take care of it and prevent its reoccurrence; do some Google searching and you’ll find lots of articles and forum discussions on the best tried-and-true- methods. Algae battles are winnable; you just need to make corrections, be patient, and observe carefully.

Good luck!


Stephen Chanco
Stephen Chanco

June 30, 2019

Growth on my untreated driftwood that look like roots. Black & stringy with a white flower looking head on top. Is this fungi or Algae?

Leave a comment