Beyond the tint: Working with water in the botanical-style aquarium.

With a burgeoning interest in the botanical-style/blackwater aquarium, and its characteristics, concepts, and functions, we're still seeing a tremendous amount of misunderstanding and misinterpretation when it comes to how to achieve more "realistic" water conditions.

The problem, as I've stated many times, is that the perception which has been picked up by the hobby is that these types of aquariums are a sort of "aesthetic alternative"- a way to 'scape tanks and create an interesting look, and maybe some "water conditions"- but the perception seems to end there. 

The deeply tinted water in many of the fantastic aquariums we see shared on social media seems to imply to many that these "tinted" aquariums feature "soft, acidic" water conditions as a matter of course- something that we erroneously assume. 

And a fair number of hobbyists, upon embarking on their first adventure with botanical materials, express frustration, confusion, and dismay that their hard, alkaline tap water is still, hard and alkaline! This type of confusion in likely cause by a lack of understanding of the fundamentals of aquarium water chemistry, and what exactly "blackwater" is. 

Understand that, as we've said many times here, botanicals (AKA "expensive botanicals" as one armchair expert referred to them recently) will not create soft, acidic "blackwater conditions" without other measures being taken by the hobbyist. 

As you likely know by now, there are a number of factors which contribute to the color of the water in your blackwater aquariums; specifically tannins released by the leaves, wood, and other botanicals you have in your tank. As we have discussed now like, 327 times (okay, maybe less...)- in many situations, leaves and other botanicals will have little to no influence on pH (unless your utilizing a water source, such as reverse osmosis, which yields  product water with extremely low mineral content and is more "amicable" to pH reduction...).

However, they will affect the color and in some instances the visual clarity of the water. 

And color generally has absolutely nothing to do with the pH of the water, really.

I really want to see less of those, "I added a bunch of catappa leaves and seed pods from that vendor on eBay, and my water is a dark brown color, but the pH is still 7.6! What gives?" sort of questions that populate online forums worldwide.

"Soft, acidic 'blackwater' conditions" seem to be the goal of many who play with botanicals. Yet, there is so much bad information out there- even after more than 5 years of us sharing everything we have learned about this unique style of aquarium with our global community.

So, I thought it would be helpful to create a series of periodic pieces here to discuss how we can facilitate the creation of such conditions in our tanks. Now, I'm not going to be giving you a "recipe" to achieve 5.2ph and 0 dkh, or something like that. No. Rather, we'll touch on some of the things that you as a hobbyist can do to help set up your aquarium to achieve such conditions.

First off, it all starts with your water.

If your goal is to manipulate the pH of your tap water to bring it from neutral or alkaline conditions to the acidic range, you need to make it "malleable." The easiest, most cost-effective way is to utilize water which has been deionized. This is based achieved with a reverse osmosis/deionization ("RO/DI") unit. The idea of deionization (AKA "demineralization") simply means that the removal of ions from the water has occurred.  Ions are electrically charged atoms or molecules found in water that have either a net negative or positive charge.

Ions which have a positive charge are called "cations" and ions with a negative charge are known as "anions". Materials known as ion exchange resins are used to exchange unwanted cations and anions with hydrogen and hydroxyl, which form pure water (H20), which is not an ion.

Okay, head spinning yet? Some of the ions commonly found in tap water include stuff like Calcium (Ca++), Magnesium (Mg++), Iron (Fe+++), Manganese (Mn++), Sodium (Na+), and Hydrogen (H+). These are removed with cation resins. Substances which we classify as anions include compounds like Chlorides (Cl-), Carbonates (CO3--), Nitrates (NO3-), Sulfates (SO4--), and Silica (SiO2-), which-wait for it- are removed with anion resins.

Deionization is a precess in which source water (from your tap) is forced through a  series of "ion exchange resins", which are small plastic beads that are composed of organic polymer chains which have charged "functional groups" formulated into the resin bead. Each "functional group" has either a fixed positive or negative charge. Different ions are attached to these types of resins with differing strengths. It's a fascinating process which I'm probably not doing the best job of explaining. 

Of course, that's not the whole story behind water treatment for our aquairums. Deionization is usually combined with reverse osmosis. Reverse osmosis is a process which removes contaminants from tap water when pressure forces it through a semipermeable membrane. Typically, a prefilter removes sediment and chlorine from water before it forces water through a semipermeable membrane to remove dissolved solids.Water flows from the more concentrated side (which has more contaminants) of the RO membrane to the less concentrated side (which has fewer contaminants) to provide "product" water. 

One your tap water has been treated in this manner, it's much easier to manipulate the pH through variety of means, including utilizing leaves, seed pods, and other botanical materials. It's the important first step towards creating "blackwater" in your aquarium.

Okay, whew! That's a lot of stuff to take on, right? And, me not being a chemist, I'm probably not doing the best job of explaining this stuff...

Suffice it to say, a combination reverse osmosis/deionization unit is a fundamental piece of equipment and great investment for your hobby. If you're serious about creating optimum conditions for your fishes, the first step is to incorporate RO/DI into your aquarium practice.

And that brings us back to...Nature.

Blackwater differed significantly from white waters in their ionic composition, and are typically more acidic pH, with low concentrations of calcium, sodium, potassium, and magnesium. They typically have low electrical conductivity as well. 

In general, blackwaters originate from sandy soils. High concentrations of humic acids in the water are thought to occur in drainages with what scientists call "podzol" sandy soils. "Podzol" is a soil classification which describes  an infertile acidic soil having an "ashlike" subsurface layer from which minerals have been leached. 

Soil geology is one of the most- if not THE MOST- profound influences on blackwater in natural aquatic systems. It plays a fundamental role in creating the chemical characteristics which are common to these ecosystems.

Also interesting to note is that fact that soluble humic acids are adsorbed by clay minerals in what are known as "oxisol" soils, resulting in clear waters."Oxisol" soils are often classified as "laterite" soils, which some who grow plants are familiar with, known for their richness in iron and aluminum oxides. I'm no chemist, or even a planted tank geek..but aren't those important elements for aquatic plants? 



In studies of Amazonian blackwater environments, it was discovered that the pH in some areas was as low as 3.5...or less...and that these habitats had significant and rich populations of fishes. Now, the fishes have obviously evolved to thrive in these environments, so it's interesting to think about what makes 'em tick, as they say.

It's thought by some scientists that the humic acids from soils and botanical materials (like leaves) that are abundant in these waters can only bring down pH so far. A current theory postulates that about 85% of the "work" in lowering pH in these waters is accomplished by the organic acids, and the remaining CO2 fermentation taking place in deep leaf litter/botanical beds, perhaps producing stronger acids, like acetic acid. Further, the dominant decomposers in these extremely low pH environments (pH 2.8-3.5) are fungi, as opposed to bacteria.


Now, I am absolutely not suggesting that we attempt to achieve ph of 3.5 by packing a 20 gallon aquarium with 18 inches of leaf litter and letting it "ferment" in our tank...I can see the fucking drama that would create now! What I am suggesting is that we consider the fact that what we consider "low pH" for aquarium environments is not all that "radical."

And further, that it's not terribly difficult to use RO/DI water to keep the low pH in a reasonably tight range in the aquarium. We need to understand the dynamics of the system...Reminds me a lot of saltwater or African Rift Lake cichlid systems...Understanding the "operating system" of the environment and working with it to keep stability and consistency.

We have a lot more stuff we can and will talk about- even in regards to water. 

And other topics, of course. Little sneak preview of a future installment in this series:

Without getting really into the weeds here, suffice it to say that if your goal is to create and maintain an aquarium with a low pH, you probably want to avoid substrates which are high in calcium. In addition to our own substrates, we utilize materials which have little to know buffering capacity or influence on pH, such as silica sand, and several commercially-available substrates which don't impact pH.

Again, creating a "blackwater aquarium" is far more complex than simply throwing in some botanicals, watching the water change color, and telling yourself that you did it! It requires a fundamental understanding of basic stuff, like water chemistry biology, and ecology- and the basics of aquarium keeping. Stuff which, I'm sad to say, still seems to mystify or elude many hobbyists, who'd much rather look at cool pics of blackwater tanks on instagram, or consult a very simple infographic.

We can do better. And we should want to...It's not that hard! And in fact, you'll find that the pretty pics of tanks we all love some much will take on so much more meaning when we understand the function and some of the science behind them.

Stay informed. Stay curious. Stay diligent. Stay bold...

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


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