The low down on the way down...

"There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy." - Hamlet Act 1, Scene 5 

Yeah, that just happened. I just went Shakespeare on you. 

For a good reason. There are still many things I the blackwater/botanical-style aquarium that we collectively don't understand, or perhaps have misconceptions about. And there are a few, more technical aspects of our fascination with blackwater which can even be a bit intimidating- even a bit "over many of our heads"

Well, definitely over MY head, anyways! 

I regularly field a fair number of questions from hobbyists who add a bunch of botanicals and leaves to their aquarium filled with tap water, and are surprised and often frustrated that the pH and hardness are unchanged, despite the "tinted" look of the water. There seems to be a collective misunderstanding about the "capabilities" of botanicals; many "powers" have been ascribed to them: For example, the idea that leaves and botanicals can soften hard water.

They can't.

Period. Nothing to discuss. We've covered this before a few times, but for some reason, the myth persists that adding catappa leaves and pods and other botanical materials will create instant nature-like conditions in your hard, alkaline, city tap water.


Oh, now sure, tannins and humic substances are imparted to water, even hard, alkaline water. Even brackish water. So, yeah, it looks all dark and tinted, even though the pH and hardness might be quite high.

That's a different subject.

To understand why, we need to all re-familiarize ourselves with the concept of carbonate hardness.

Now, before we get too far, I'll dispense with the necessary disclosure that my knowledge of water chemistry is quite basic, and I'm not preferring that anything discussed here is the "last word" on the subject. It's an explanation of some facts and ideas based on my limited college chemistry and understanding of these things from being a practicing aquarist. At a certain point in discussion about this stuff with really knowledgable people, my eyes start to glaze over...There are plenty of you out here who could "school me" on this stuff, and I encourage your input on these more esoteric, yet very important aspects of the hobby. We will all benefit.

Ahh, back to that bit on carbonate hardness...

This is one of those terms, along with "general hardness" (GH), that we see bandied about all over the internet and in books and hobby discussions...It's super-confusing to me, as there are multiple ways of determining the hardness of water (in general, but for us it's for aquarium purposes). "Hardness", is essentially a measure of the total concentration of specific minerals dissolved in the water, including calcium and magnesium, as well as other minerals like potassium and sodium. It is said that the concentration of these minerals in a given quantity of water contribute to the "hardness."

There are a few ways of measuring this.

As a reef hobbyist, I was long ago indoctrinated to utilize KH (from the german word "karbonate") to measure the carbonate and bicarbonate ions in a given aquatics system, which function as "buffers", and keep the pH from dropping. And KH is a component of GH, to make matters more confusing (KH can never be higher than the "general hardness" of the water because of this fact). And a lot of test kits will measure if a guy like myself needs more confusion in his life...

The easiest way to make "hard" water "soft" for us fish geeks is to invest in a reverse osmosis/deionization unit (RO/DI unit). This gives you water with little to no (ideally) general hardness, setting the stage to more easily reduce the ph utilizing botanicals and such.

And of course, that's where the fun (and confusion) begins!

A lot of people ask about utilizing leaves and other botanicals to lower the pH in their aquariums. It's a good question, and one worth addressing. As you are no doubt aware by now, many of these natural materials release substances such as  tannic and humic acids into the water, which can acidify it- IF the water has a low enough KH.

I'll say it again:

Most botanicals won't do much to significantly reduce the pH if you start with hard, alkaline water, as the KH will prevent the acids released by these materials from reducing the pH. In general, it's fairly safe (gulp) to state that soft water is usually acidic, and "hard" water is usually alkaline.

And with a soft-water, low pH aquarium, you really have to think about stuff like the nitrogen cycle, too, and how it functions at very low pH levels.


I'm also curious, as many of you are, about the long-term "function" of what I'd consider really low (from an aquarium standpoint, anyways) pH 5.5 and below. I personally have used straight RO/DI water for years in my systems with lots of botanicals , and do use a substrate (sand),even when I mix in botanical materials, and my pH is almost "pinned" in the "mid sixes", so my hypothesis is that there is at least some buffering taking place in the tank from carbonates that might be the sand.

The bacteria which perform denitrification really function optimally at around pH 7.0 (neutral), and as the pH goes down, their performance significantly decreases. When you get below a pH of say, 6.0, into the "fives", it's safe to state that the bacteria are barely functioning. In those really low pH tanks, chemical filter media (which remove ammonia directly) are your best option. We're talking about commercially-available zeolites and such. These materials need to be exchanged regularly to function optimally and remove ammonia.

Now, it's not "disaster time" when you get into this range- it just requires greater understanding and a different approach to nitrogen cycle management. Taking the time to learn about the arena in which you're playing. Learning the rules and dynamics, and adjusting your practices to accommodate the requirements dictated by these parameters. Understanding the impact of the materials you put into your aquarium water.

Or, as one of my buddies so eloquently put it during one of those alchohol-fueled fish conference discussions some years back, "The idea is not to kill fish with this shit..." Yup. You don't "dabble" in very low ("aquaristically-speaking") pH systems-or any specialized aquatic system, really- without a game plan. Oh, and a pretty good understanding of chemistry- like, way better than what I have. 

I guess it could be safe to say that, in the majority of my systems, I play in "the higher end of the lower pH range", if that makes sense! Quite frankly, this pH range has satisfied me, and my fishes seem to benefit from it. I personally have not experienced any nitrogen cycle issues, pH "crashes", or any other scary things while managing my tanks in this range. 

I suspect (hope) that many of you are also operating in the "sixes" in your systems. I personally have never attained super-low pH (like in the "fours") in my systems, even when using "straight-up" RO/DI and just botanical materials for a substrate (no sand). Now, your results might vary...but that's my experience.

That being said, I think there is  plenty of room for responsible and disciplined experimentation here by those interested and knowledgable in the subject. I was talking with Ted Judy some time back, and he was talking about techniques utilizing acid solutions to really get the pH down way low in systems housing fish like wild Altum Angels and such; the thought being that ammonia is bound up as less toxic ammonium at those low pH values...and these guys can be pretty touchy fishes when first imported, as we know. Interesting idea in responsible, patient, experienced hands!

Now, this is pretty cool stuff, and quite frankly, beyond my particular scope of interest and even my skill set; however, it certainly is relevant to this stuff! And interesting! To me, adding pH-reducing acids to get down into those low ranges is serious business, and quite a bit out of my personal comfort range- especially NOT being a "chemistry-minded type!"  

And of course, most fish are utterly intolerant of significant ph swings and you have to be very careful (and slow!) if you attempt to make large changes to the chemical composition of your water in this fashion. I'd dare say that this type of pH manipulation falls in the "experts only" category for now.

So this leaves most of us in a position of doing what we're already doing: Managing our soft, acid (in the "sixes") water aquariums in a manner consistent with good husbandry, going slowly when adding botanicals, and generally testing and observing our fishes. Ours is a world of balancing too much- and not as much as we want- a world of observation, measurement, continuous self-education and experimentation.

We can't be "casual" or "seat-of-the-pants" when we get to the lower pH ranges. It's not "set and forget"...Active management is required. We don't enter this arena lightly, and I think we are all aware that each and every blackwater, botanical-style tank requires thoughtful husbandry and a generalized understanding/consideration of water chemistry.

Disasters occasionally happen, as we've reiterated countless times, especially when we go too fast, or make too many changes to existing aquariums in a compacted time frame. Restraint and patience are mandatory. And an understanding that taking new and different approaches always involves risk and a learning curve.

It's part of the game.

However, I do think we're getting beyond the "dump and pray" part of this, and finding what we consider to be reasonably good compromises in terms of safety and benefits. We're developing and refining knowledge and technique. More and more hobbyists are playing with blackwater, botanical-style aquariums every day, and unlocking the secrets, demystifying some of the fears, reiterating the cautions, and sharing their knowledge of these unique systems with the aquarium world.

The "way down" to lower pH can be intimidating, filled with unknowns, cautionary tales, and spotty information. However, it doesn't have to be, if you approach it with the correct mindset.

And that mindset is to learn as much as you can, and incorporate what you've learned into your aquarium practice. Then, simply move forward. Carefully, of course.

Don't be afraid. Be empowered.

Be cautious, bold, informed, observant, and methodical.

Stay educated. Stay careful. Stay experimental. Stay in control...

And Stay Wet.


Scott Fellman

Tannin Aquatics 

Scott Fellman
Scott Fellman


Leave a comment