"That pH thing...again."

Oh, it's back to the pH thing, and how botanicals affect it again?

Yeah, it is...

Well, I think we need to talk about it, because we receive so many questions about this topic and there seems to be a lot of misunderstanding about it.

The reality is that botanicals CAN influence the pH of your water...under certain circumstances. 

Simply adding catappa or other leaves to your hard, alkaline tap water will have little to no effect, other than to impart some color and maybe increase your TDS a bit.

I'm starting with RO/DI water, with essentially no carbonate hardness and a very "flexible" pH. Not only do the botanicals influence the color, but they can influence the pH under these circumstances by virtue of the humic acids they can impart to the water. 

However, the tannins, which are the substances which "tint" the water, cannot "overcome" the calcium and magnesium ions, and drive down the pH significantly in water with high levels of these ions present. It simply is putting more materials into the water (which are often detectible by TDS meters in aquariums).

Remember, there are multiple factors in play, and multiple goals you might have for adding leaves to your tank. If it's just about the aesthetics of having leaves in the tank, and you want clear water, use activated carbon in your filter and call it a day!

If you're all about a deeply tinted and low pH environment, you need to eliminate activated carbon in large quantities, employ RO/DI water, and likely use quite a bit more of these materials than I am talking about here.

You have to experiment.

There is simply no "recipe" out there that can give you "Instant Amazon" conditions by adding "a little of this and a bit of that" to your tank, despite what "experts" or vendors will tell you. There just isn't. Period. 

I could talk until "the cows come home" about how much of "this and that" I use in my tank- but I may have a combination of factors that are vastly different from the environment in your aquarium. Sure, you can get some inspiration and a general idea- but your results will vary.  Every tank is different. I think it's more about understanding the general "practices" involved and their impact, versus simply learning "how many" of "whatever" to add.

Until we as a hobby community back off from looking for some specific "recipe" on how to achieve "x" water parameters without exception, and look at each aquarium as a sort of "one off" situation requiring a custom solution, confusion and misinformation will continue to proliferate.

Sure, I can give you my "blackwater recipe", but it's just that: My blackwater recipe.  It works for me:

In one of my display tanks in my home, I use a mix of Yellow Mangrove and Texas Live Oak, with a bunch of Red Mangrove Bark and Oak twigs to achieve a nice color that I like.  (notice I talked about the color in the context of the materials I use?). Now, I also have a pH of around 6.5 and undetectable carbonate hardness; a TDS of around 6 (interesting....).

But that's me. 

I like it. And my fishes like it. My tank runs in a manner that works for me and my fishes. It's not trying to replicate a 4.8 pH Amazonian stream or whatever. It's about creating and managing an environment which I can easily and effectively maintain.

Oh, and the idea of utilizing chemical filtration in blackwater tanks...

Now, I've already touched on the issue of chemical filtration and its impact on removing the tint and tannins associated with leaves and botanicals a number of times in "The Tint" over the years.

Yes, it's entirely possible to minimize or render the ph-lowering and water-tinting capability of tannins released by leaves with activated carbon, Purigen, or other chemical filtration media. Tannins are rather weak acids, yet they can lower the pH of water when their is less "buffer" in the system (i.e.; lower general hardness). Just how much tannins can lower pH in a given system depends upon how much buffering capacity the water has. 

With "harder" water (i.e., water with a greater buffering capacity), you can have the tinted water look from leaves and wood, without the pH reducing effects, particularly if the acids are absorbed by the aforementioned chemical filtration media. So, in other words, you can have the aesthetics of blackwater while running your aquarium at a higher pH if the hardness is sufficient. Once you remove tannins in a lower hardness system, your pH should rise, too, since you're removing the acids.

Woah. Head spin time. 

Bottom line is this: You can use chemical filtration media in "tinted" tanks. However, depending upon the amount of media, quantity of tannin-producing items (leaves, wood, botanicals, etc.) and the capacity of the aquarium, the impact will be variable.

I run chemical filtration media (Purigen and activated carbon) in my tanks, and I still have nice tint and pH relatively stable, as mentioned above. It's a fine line between "too much" and "too little", and you will simply have to experiment to find what works best for you! Nuance and testing guide you.

And never let yourself be fooled. It's a fact:

Botanicals and leaves will NOT soften your water.

I think it's perhaps the most misunderstood thing of all about botanicals in our aquariums. And, I suppose it's easy to see how this one got started and tends to hang around a bit:  (Yes, redundancy tie. again!) Most botanical materials contain tannins and humic substances, which can drive down the pH in water with little to no carbonate hardness.  

And of course, the tinted, soft acidic water in many natural habitats often has an abundance of leaves and botanicals present. I think that this gave a lot of hobbyists the impression that you could simply add some of these materials (leaves, etc.) into your tap water and create "Rio Negro-like" conditions easily! Many are convinced that the "look and color" are indicative of the chemical composition of the water...a misleading and erroneous assumption, as we've discussed repeatedly.

Remember, this can only be accomplished in the aquarium by utilizing source water which has been treated via reverse osmosis or ion exchange ( a process in which calcium and magnesium ions are "exchanged" for sodium or potassium ions.)

Reverse osmosis is a water treatment process which relies on a membrane which has pores large enough to admit water molecules, yet "hardness ions" such as Ca2+ and Mg2+ remain behind and are flushed away by excess water. The resulting product water is thus called "soft water"-free of hardness ions without any other ions being added. 

Want to easily create soft, acid water? Get an RO/DI unit and be done with it...

I attribute my aquariums' relatively stable soft, mildly acidic conditions to the use of reverse osmosis/deionization (RO/DI) to pre-treat my tap water. Yes, RO/DI units are a bit pricy at first. However, IMHO, they are an essential piece of aquarium equipment and a very wise long-term investment for the serious natural aquarium hobbyist!

Yes, this article may simply make your head spin just a bit more, and I apologize if it does...

However, the real aim was to help clear up a few assumptions about creating the type of water conditions in our aquariums which replicate, on some level, some of those from the natural habitats from which our fishes come. Maybe, to get you looking at more of the facts and science behind this stuff, rather than simply searching for a product or "recipe" on how to do things.

I hope that we've helped just a bit!

Stay curious. Stay studious. Stay open-minded. Stay resourceful. Stay diligent...

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


4 Responses

Scott Fellman
Scott Fellman

July 17, 2022

Hi Kendall- yes- you hit this on the head…The buffering capacity of the tank is what’s making that pH shoot back up…That and other possibilities, such as the acids being removed by filter media, being bound to substrates, etc. I typically use straight RO/DI in my tanks without any mineral reconstutition, and I use non-pH-impacting substrates (like silica, etc.) and I do not experience these types of pH “rebounds”…

I love that you’re experimenting and revisiting information- it’s the key to success and really puts you in an elite class of hobbyists, imho!



June 07, 2022

Hello Scott, I’ve been reading this article with great curiousity as I have been trying to lower the PH of my Corydoras Eques breeding setup to around 6.8-7.2 in the last couple of weeks. I used to run the tank (with C. Eques in it) on tap water (PH 8.1, TDS ~300ppm.

I have an RO-unit and have been doing three 30% water changes with RO water for the past week. PH dropped to 7.5 and TDS is now at ~90ppm. I did some experimenting with pure RO and tannins (boiled blackwater leave mix) and noticed adding 0.2% of these botanicals gave me a PH of 6.2, 2.0% gave me a PH of 4.9. So, these measurements gave me something to work with.

So, last night I added about 1% of tannins to the breeding tank and initially the PH dropped from 7.5 to 7.0 within 30 minutes after adding the tannins. However, the next morning PH was up to 7.3 again.

From this article I take doing more water changes with RO would lower the water’s buffering capacity and thus allow tannins to lower the PH more efficiently after that.

Is my assumption correct?



Scott Fellman
Scott Fellman

January 15, 2021

Hi Kendall,

Thanks for the kind words!

I really appreciate your being conscientious and realistic about what botanicals can do with regards to pH. I think that somehow, the hobby has been enamored with the “look” of blackwater/botanical-style tanks, assuming that because of the color of the water, it’s somehow “soft and acidic…” Realistic observations like yours will go a long way towards helping to dispel those misunderstandings.




January 14, 2021

Scott, another excellently written and pointed article. I’m currently trying to lower the pH in my Walstad cube garden from ~7.6 to 6.5~7.0. My first thought was to dump catappa leaves into the tank until the pH started to fall. I clearly overlooked the buffering capacity of my not so soft water. Upon introducing 10 leaves to the 27L, the pH did fall roughly 0.2, however as the pH scale is logarithmic, I want to be very cautious not to shock the now 2 year old tank.

Thank you for the insights, and maybe it’s time I reread Diane’s wonderful text, and look before I leap.

Kind Regards,


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