Blackwater "on the down low"- lessons to learn and repeat...

It seems like the deeper we delve into the blackwater, botanical aquarium world, the more ideas we get from nature, the more questions we have, and the more things we discover that are worth considering for our "practice." 

For example, the extremely low pH environments from which blackwater fishes come from are a very demanding environment, not to be taken lightly. However, our fear of very low pH aquariums is probably largely grounded in concerns over fluctuations in the parameters, and the challenges of maintaining consistent environmental conditions. 

Now, I'll agree, going into extremely acidic pH ranges is something we as a hobby have not had tremendous experience with, yet I get that nagging feeling in the back of my head that at least part of our fears may be unjustified. I mean, we've already discussed that ammonia at lower pH levels is not toxic. This, I feel, is where the bulk of the "mainstream hobby freak out" against our blackwater/low pH tanks stems from, so it deserves another look. As hobbyists, we should understand what we call "TAN"- or "total ammonia nitrogen." Our hobby test kits measure both ammonia (NH3) and ammonium (NH4+, also called "ionized ammonia"), which comprise "TAN."  This has important implications for those of us who maintain a low pH system ( like, below 6.0).

We need to be careful in managing our low pH, blackwater aquariums- particularly when performing water changes. Here's why:  Low pH can have a detrimental affect on the population of nitrifying bacteria that converts ammonia to nitrite.  Because of the acidity  of the water, these bacteria populations can drop so low that the "total ammonia nitrogen" (TAN) level can rise quickly.  No cause to freak out, because if the pH stays low the TAN level is pretty much all ammonium (NH4+), which is the essentially "nontoxic" ("gulp" for saying that!) component of TAN.  (To make you feel better, ammonium, even at high concentrations, doesn't kill fishes!)

If you do water exchanges with lots of alkalinity buffer added to the makeup water, the pH goes up, right? And what happens at a higher pH level? You guessed it:   The "non-toxic" ammonium can be quickly converted to very toxic ammonia, potentially causing ammonia poisoning to your fishes. I will hazard a guess that this is where many hobbyists have screwed up, and that many the "anomalous deaths" in otherwise "well maintained" low pH tanks were caused by this situation...and that further added to the "lore" that suggests that low pH, blackwater-type aquariums are bad news!  Key takeaway here: Keep things stable. If you're going to keep a low pH, KEEP it that way. Use water for exchanges that has pH values consistent with the water in your aquarium.

And that brings us back to...nature.

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 botanicals 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 drama 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.


The other interesting thing about these extremely acidic habitats from the studies I read were that the other parameters, like temperature, oxygen saturation, etc. were quite stable (oh, and not surprisingly, that the ionic content of the water was extremely low, as well). Again, stability within a range seems to be the predominant factor in the function of these environments, and this is a direct lesson for us as aquarists. It seems that no matter how "radical" the environment might seem to us, keeping the parameters stable wins out.

Although most of us are not too likely to explore extremely acidic pH levels in our aquariums, the lessons learned from nature on the function of these habitats can help us manage our "low pH" systems (like 6.5-4.9) much more effectively. There is still so much we don't know yet. We know that there are different types of bacteria and other microorganisms which take over for the more familiar Nitrosomanas and Nitrobacter at these lower pH ranges, and if it were possible to culture some of these organisms for use in filtration media for aquariums, can you imagine the impact for hobbyists who ply their trade in these waters?

I'm thinking Altum Angelfish guys, wild Discus keepers, etc.

Lots of possibilities here.

Like everything else, just contemplating these things leads to more questions than answers, but it shows you how much more there is for us as hobbyists to learn and understand, particularly from the wild habitats. Watching our fishes and observing OUR aquariums creates unique opportunities to break away from the dangerous "groupthink" that has, in my opinion, held the hobby back in recent years. Keeping an open mind and gently questioning "why" stuff is in our hobby will help us move beyond any previous restrains we have had.

Look, we will all make mistakes, go down some wrong directions, and possibly be extremely humbled by what we learn along the way. However, we'll be learning and doing- first hand- and not just accepting what is considered general hobby norms and practices "because."

Exciting times. Who's up for researching this stuff?

Stay excited. Stay curious. Stay relentless.

And Stay Wet.

Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


2 Responses

Scott Fellman
Scott Fellman

February 03, 2017

Ohh…love the idea, John! Very analogous to the “Remote Deep Sand Beds”used in reef tanks! I absolutely think it’s worth investigating…Or what about a “leaf reactor”, like a very large fluidized reactor pack with leaf litter…I wonder if it would turn into a hydrogen sulfide bomb, or actually foster some sort of fermentation process? Hmm..I envision the stinky hydrogen sulfide thing, but…:) Giving me ideas for another blog! -Scott


February 02, 2017

In the subject of 18" of leaf littler being left to ferment. I wonder if you could have a simp with a section for this to occur. Just a thought, let me know what you all think.

Leave a comment