The cure?

One of the typical "best practices" of modern aquarium keeping is the "preparation" or "curing" of wood before adding it to your tank. There is very little questioning within the hobby about the need for some preparation before it's "suitable" for keeping with fishes. 

We're admonished not to simply toss wood into the aquarium. It makes sense, right? I mean, being a product of the terrestrial environment, wood can contain a lot of dirt, dust, and other pollutants which, upon wetting, will leach into the water. And doing at least a thorough rinse with fresh water is a practice that I absolutely agree with.

When you first submerge wood, a lot of the "dirt" from the atmosphere and surrounding environment comes off, along with tannins, lignin, and all sorts of other "stuff" from the exterior surfaces and all of those nooks and crannies that we love so much.

Ahh, the tannins again.

Now, I don't know about you, but I'm always almost sadistically amused by the frantic posts on aquascaping forums from hobbyists that their water is turning brown after adding a piece of driftwood. I mean- what's the big deal?

Okay, not everyone likes it...I get it. But it IS kind of funny to me, lol.

However, when it comes to one of the main reasons why we're educated from day one in the hobby to "cure" wood is that it will leach an abundance of tannins into the water upon submersion, and that most wood types will continue to leach tannins pretty much for as long as they're submerged. As a "tinter", I see this as a great advantage in helping establish and maintain the tinted "look", to foster the microbiome, and to impart the humic substances that have been proven to be very beneficial to the health of almost all freshwater fishes.

It's a unique aesthetic, too, of course!

When it comes to preparation, I'm really more concerned with those impurities- the trapped dirt and such contained within the wood, as opposed to stuff like lignin and tannins.

As you probably know by now, that's also why I've been a staunch advocate of the overly conservative "boil and soak" approach to the preparation of botanicals, too.

When it comes to wood, a lot of material gets bound up in the dermal layer of the tree where the wood comes from. The bulk of the dry mass of the xylem (the "network" within the tree which transports water and soluble mineral nutrients from the roots throughout the plant, and comprises what we know as "wood.") is cellulose, a polysaccharide, and most of the remainder is lignin, which is a sort of complex polymer. 

Why the "mini botany lesson?"

Well, because when you have some idea of what you're putting into your tank, you'll better understand why it behaves the way it does when submerged! In a given piece of driftwood, there is going to be some material bound up in these structures, and it will be released (gradually or otherwise) into the water that surrounds it, often with a big "burst" occurring upon initial submersion. 

This is why, during the first couple of weeks after you submerge wood, that the water often becomes dark, and even cloudy. There is a lot of "stuff" in there!

That's why it's long been recommended as a "best practice" to start the "curing" process in a separate container apart from the display aquarium. This is not rocket science, nor some wisdom only the enlightened aquarists attain. It's common sense, and a practice we all need to simply view as necessary with terrestrial materials like wood and botanicals. You may love the tannins as much as I do, your tank could do without the polyscaccharides and impurities from the outer layers of the wood.

The potential affects on water quality are significant!

"Significant" doesn't necessarily mean "bad", though

Here is a natural corollary: It's pretty plain to see that at least part of the reason we see a burst of new algae growth and biofilm in wood recently added to an aquarium is that there is so much stuff bound up in it. Algal and fungal spores can literally "bloom" during the initial period after submersion. It's exactly what happens in the wild aquatic habitats of the world when tree trunks and branches are covered by water.

A lot of hobbyists simply don't want to see this stuff in their display tank.

On the other hand, the adventurous aquarist in me can't help but wonder if we should just give the wood a thorough washing, and let this whole process play out in the aquarium, to foster this amazing biodiversity within the aquarium itself.

Again, this is an example of setting up an aquarium from the start to replicate both the form and function of Nature.

Why NOT do this? What would the downsides be? I've done this a bunch of times with no issues. However, the experience IS a bit different.

It's starts with what you see.

Yeah, you'll see a lot more biofilm, fungal growth, detritus, and perhaps even slightly hazy water. You'll have to carefully monitor the nitrogen cycle, and manage nutrient accumulations with good husbandry...

You'll have to employ a lot of patience, and yeah, I'd recommend testing during the "break-in"process. Testing for what? Well, I'd likely do ammonia and nitrite, for starters. "Have you done all of this testing when you tried this, Scott?"

Not always, I admit. Why? For one thing , it's because I'm in no rush to add fishes to brand-new tanks. Because I let my tanks develop biologically for a long time before I add them. I did out of sheer curiosity, of course! And the "cycle"time was really nothing extraordinary at all.

Really, the biggest difference between this "in-tank-curing" and using an external container was that any of the stuff that emerged from the wood itself would accumulate in the display tank, and impact the water appearance, and chemistry. Although I admit, I didn't notice a significant difference in nitrate or even phosphate in new tanks where the "curing" process was undertaken internally.

Remember, I'm a water exchange fanatic; I perform 10% water exchanges in every tank I maintain- every week, without fail.  So there was some dilution of whatever organics were found in the water.

The biggest difference determined by testing was often TDS. And of course, because TDS represents the total concentration of dissolved substances in water it can include both inorganic salts, as well as a small amount of organic matter. To me, "TDS" is always a bit of a vague thing; I mean, it can be so many different things. Regardless, when I cured "in situ", TDS readings were higher than in tanks where this process wasn't employed.

Do some of the other materials leached out of wood have implications for the healthy break-in and operation of your aquarium?  Can you even test for everything that leaches out of newly submerged wood, other than simply labeling these compounds as "organics?"

Well, lignin is one substance that you might find leaching out of wood. And there are actually lignin test kits out there for scientific work; I suppose it would be interesting and informative to test for them to see what the concentration was, although I'm not really sure what function it would perform, other this just kind of "knowing."

Just like with testing for tannins, Interpreting what is "baseline" or even "okay" for lignin is something we have never really done in the hobby, right?  Another supposition would be that lignin concentration might be different in a filtered aquarium than it would be in some big container of water without a filter that you might cure wood in.


The point is, there are some things that we just don't know. We assume. I Mean, whenever we "cure" wood externally, we almost always see lots of that yucky biofilm and fungal growth on the surface tissues. That's "par for the course" when terrestrial materials are submerged. The real issue that makes "in situ" curing a bit unusual is the possible "gross pollutants" that may leach out of the wood. I suppose that would be stuff like dust, dirt, maybe some small amounts of sap, etc., bound up in or on the surface tissues of the wood.

And of course, the tannins.

It begs the question, once again, about the dangers associated with this process. Are they perceived, or actual? I mean, sure, an initial spray and scrub, or even an overnight soak, is absolutely recommended, even if you're curing in the tank. I also inoculate my tanks with bacteria, such as our Purple Non-Sulphur bacterial additive, "Culture."

If you really want to dig down on the idea, ask yourself this:

What leaches out differently in the display tank than it does in an external curing container? Likely, nothing... The main difference, of course, is that you have these compounds releasing into what ultimately becomes your display tank.

So the initial water quality is impacted while this process plays out...How long does it last? It can be as little as a few days up to a month or more. I say, who cares? We're not in any rush to add fishes to our aquariums, so why is this a problem? We view all of this material as "fuel" for the microbiome we're trying to foster- it's actually a contributor to the ecology of your tank! 

Our work with the "Urban Igapo" concept and it's use of sedimented substrates, roots, etc. has sort of "mentally conditioned" us to accept this process and its cadence. Dealing with "alternate aesthetics" for a period of time is not exactly alien to us.

I did a lot of research on this in the online forums, articles, etc, and the reasons why it's recommended that wood be "cured" outside of the display tank are always listed as (in no particular order):"to leach out impurities", "to leach out tannins", to "let the fungal growth subside", and "to waterlog and sink."

Now, other than "waterlog and sink" process, which you can accomplish in the display tank by simply placing a few rocks on the wood, IMHO none of the other reasons given for external curing of wood are really "non-starters" here.

It's occasionally stated that boiling wood or extended soaking helps eliminate potential parasites that might be present in/on the wood. I'd hazard a guess that most wood used in aquariums doesn't have significant populations of parasites that could harm fishes, either. And again, even if there are such parasites present, if you're taking your time to add fishes (essentially keeping your tank "fallow" for a period of time) you're essentially denying any parasites that are present their "hosts", right? 


Am I missing something here? 

I don't really think so. It's just that I don't see the "stuff" that happens during the curing process as a problem.

"In situ" curing isn't a perfect, guaranteed route to accomplishing everything you want to easily, but it works. And the process and its impacts on the ecology of your aquarium is not all that different than what occurs in Nature, when you think about it.

In Nature, it is not uncommon at all for small (and large) trees to fall in the rain forest, with punishing rain and saturated ground conspiring to easily knock over anything that's not firmly rooted!


When these trees fall over, they often fall into small streams, or in the case of the varzea or igapo environments in The Amazon ( the ones that I'm totally obsessed with), they fall and are ultimately submerged in the inundated forest floor when the waters return.

And of course, they immediately impact their (now) aquatic environment, fulfilling several functions.

Fallen trees provide a physical barrier or separation from currents, perhaps creating a little "dam", which accumulates leaves, sediments, and detritus- all important as food sources to a huge number of aquatic organisms. 

They also provide a "substrate" for algae and biofilms to multiply on, and providing places for fishes forage among, and hide in. Many fishes, like small cichlids, will reproduce and raise their fry among these fallen tree trunks.

An entire community of aquatic life forms uses the fallen tree for many purposes. And the tree trunks, branches, and other parts of the tree will last for many years, fulfilling this important role in the aquatic ecosystems they now reside in each time the waters return.

Let's focus on this "ecological component" for just a bit. Let's review what happens when a tree falls...literally!

Shortly after falling into the water, fungi and other microorganisms act to colonize the surfaces, and biofilms populate the bark and exposed surfaces of the tree.  Over time, the tree will impart many chemical substances, (lignin, humic acids, tannins, sugars, etc.) into the water as the bark breaks down and the tree itself softens.

In aquatic ecosystems, much of the initial breakdown of botanical materials is conducted by detritivores- specifically, fishes, aquatic insects and invertebrates, which serve to begin the process by feeding upon the tissues  while other species utilize the "waste products" which are produced during this process for their nutrition.

In these habitats, such as streams and flooded forests, a variety of species work in tandem with each other, with various organisms carrying out different stages of the decomposition process.

The fallen tree literally brings new life to the waters.

I can't stress enough how interesting and important this transformation of the terrestrial environment to the aquatic one is. It helps explain so much of why the aquatic habitats look and function the way they do, and how they impact the life forms which make use of them.

The materials that comprise the tree are known in ecology as "allochthonous material"-  something imported into an ecosystem from outside of it.  (extra points if you can pronounce the word on the first try!)  We've talked about that stuff for a while now, right?


So, to "put a bow" on the idea of "in situ" curing of wood, it is sort of analogous to this natural process, both in terms of the way various compounds are imparted into the water, and in terms of the ecological impacts it causes in the aquarium. Now of course, an aquarium is not Nature, and being a closed system, there needs to be biological processes in place to assimilate these materials. 

It's as simple- and as complex- as that.

So the idea of "curing" wood for aquarium use- while important to understand- may not be as impactful as we think, especially if we look at the process as a means to contribute to the ecological processes which occur in your aquarium. It's about perspective, as much as anything, right?

I think that so many things in the hobby that we consider "problems" are simply not problems. They require us to think critically and consider the context of what we do.

I'm curious to hear fo your experiences with this non-conventional approach! Remember, it's not "bad" unless you don't understand the implications of what you're doing. 

Think about that!

Stay curious. Stay observant. Stay experimental. Stay informed...

And Stay Wet.


Scott Fellman

Tannin Aquatics 



Scott Fellman
Scott Fellman


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