More than you care to know about Catappa Leaves...


There has been an amazing trend in the aquarium world of late; a trend towards incorporating natural materials in our aquascapes, encouraging us to try new concepts, new approaches, and new and exciting aesthetics. I'm kind of stoked to see that Tannin Aquatics is sort of pushing and prodding things along, too!

It's exciting to see the idea of leaves in the aquarium getting a fresh look. It seems that we're no longer talking about using leaves simply to create blackwater-type conditions for breeding fishes. Now, we're starting to look at them for the aesthetics and other functional capabilities they bring to the aquarium. And that is even more exciting to us!

It all starts with the Catappa leaf.

Yeah, the "vector" for the "New Botanical" movement of the hobby is the humble leaf that's hardly unknown to the hobby. Derived from the Terminalia catappa tree, this beautiful leaf has been used in aquariums for some time, particularly, by Betta enthusiasts in Asia, for its purported antifungal and antibacterial benefits.  And, current scientific research has proven this to be true. In addition, humic substances, derived from Catappa and other leaves, have been shown to provide a host of health benefits for tropical fishes. Only recently are aquarists discovering and experimenting with the many possible aesthetic applications for them as well.

 The idea of incorporating an area of leaf litter in an aquarium has been irresistible to me, and several years ago, I began to explore the ins and outs of the popular Catappa leaf and its applications and implications for aquarium use. I think my enthusiasm stems from the fact that, when properly and aesthetically employed, leaf litter adds a new dimension to a biotope-inspired aquarium, in addition to utility.


Many fishes (particularly South American fishes like Tetras, Cichlids and catfishes), as well as numerous African and Southeast Asian species (Gouramis, Bettas, etc.) benefit from the tannic acids and other substances released by these products into the water. 


The bark and particularly, the leaves of the Indian Almond tree contain a host of interesting chemicals that may provide benefits for tropical fishes. The leaves themselves contain several flavonoids, like kaempferol and quercetin, a number of tannins, like punicalin and punicalagin, as well as a suite of saponins and phytosterols. Extracts of T. catappa have shown some effectiveness against some bacteria, specifically, Plasmodium, and some parasites as well. 


When Indian Almond leaves are immersed in water, the tannins and humic substances are released, which can lower the pH of the water. The tannins are what tint the water to a beautiful brownish color, known colloquially as “blackwater.”


It has been theorized that the tannins in Catappa leaves are able to reduce the toxicity of heavy metals in aquarium water, essentially binding them up or chelating them- a most interesting benefit for the urban fish keeper, I might add! At this point, you’re probably thinking, ”Okay, Scott. That sounds very scholarly, but what exactly are those things and what can they do for my fishes?”







What makes these leaves tick?


Well, lets start with the flavonoids. Flavonoids have been shown to have direct and synergistic antibacterial activity (with antibiotics) and the ability to suppress bacterial virulence factors in a number of research studies. They may also act as chemical "messengers", physiological regulators, and "cell cycle inhibitors", which bodes well for their use as a prophylactic of sorts. Kaempferol, a noted flavonoid,  is thought to have anti-inflammatory and antioxidant properties. Could this be why Betta fanciers used them for so many years after fighting their fishes?

Saponins can be used to enhance penetration of macromolecules, like proteins, into cell membranes. Some are used in vaccines to help stimulate immune responses, so you can imagine some potential benefits here as well

Phytosterols are interesting for their alleged capacity to reduce cholesterol in humans, but the benefits are probably non-existent for fishes, especially as it pertains to Catappa leaves in the aquarium! I mention them in this piece merely because fishy authors touting the benefits of Catappa leaves love to throw them out there for reasons I cannot grasp!

Punicalagins act as antioxidants and are the major component responsible for the antioxidant health benefits of fruits, such as pomegranates (You know, the "wonderful," yet really messy fruit that I always hated as a kid...). They are water soluble and have high bioavailability, so it is quite possible that they are of some benefit to fishes!



I recall hearing about cool study conducted by fisheries researchers in Thailand on Tilapia, which concluded  that Catappa extract was useful at eradicating the nasty exoparasite, Trichodina, and the growth of a couple of strains of Aeromonas hydrophila was also inhibited by dosing Catappa leaf extract. In addition, this solution was shown to reduce the fungal infection in Tilapia eggs. 


Using Catappa leaves in the aquarium: Preparation, considerations, and cautions

So how many leaves do we “need” in our aquariums? 


Interestingly, there is not a really clear-cut answer here! We may have to rely on the completely anecdotally-derived "recommended" number of leaves per gallon as determined by long-time users of the leaves. In other words, estimates based on our intuition, what we’re trying to achieve, and ultimately, the results we're getting!

The "generally accepted" quantity for Catappa leaves is subjective, at best- typically like 1-2 large leaves (approximately 5" plus) for every 15 gallons (approximately) of tank capacity, or several smaller ones for the same tank volume. And of course, like so many things in this hobby, if you ask 10 aquarists you'll probably receive 10 different answers!



 When searching for Catappa leaves on line or from other sources, it’s very important to make sure that you’re obtaining them from a source which hasn’t used pesticides or other chemicals, which can leach into your water and have disastrous results for aquatic life. The leaves most reputable vendors sell are free of pesticides and major pollutants.

Most high-quality Catappa leaves require minimal preparation for aquarium use, yet still should be rinsed at the very least before adding to the aquarium. I like the idea of steeping leaves in boiling water for 10 minutes or so, which helps soften them up a bit and "sterilizes" them to some extent, without "cracking off" all of the beneficial tannins bound up in their tissues.

 Possibly the most important issue when utilizing Catappa leaves and other botanical materials in the aquarium is water parameter management- as it relates to soft, acidic water, environmental stability, and even its implications for the ever-important nitrogen cycle. 

 Without going into the intricate details, it's a fact that "soft" water- water with little in the way of dissolved minerals to absorb acidic compounds, is inherently more prone to more rapid and significant drops in pH than "hard" water- water with greater concentration of dissolved mineral ions- and creates more challenges for the aquarist, particularly when adding materials to the aquarium which can lower the pH of the water significantly.  When adding Catappa leaves to an aquarium with very soft, already acidic water,  prepare all botanicals thoroughly as instructed above, add them very slowly, and monitor pH /KH as you go to judge for yourself the impact of these materials on your water chemistry.


 Remember, when you're using reverse osmosis/deionized water, it's devoid of most all minerals to help "buffer" the pH from falling towards acid, so you need to "remineralize" it with commercial additives, or utilize some buffers that typically keep the pH in the mid to high "sixes" (like 6.5-6.8). This will give you some "reserve" and stability, should the pH begin its fall towards the lower end of the pH range.




One solution towards preventing this lack of stability is to utilize some soft water with your hard water from the tap (dechlorinized, of course), to get the hardness down to a manageable and safe range of 5-10 degrees of hardness. There are formulas out there to tell you how many parts of each you need to mix to achieve a specific hardness (of course, you need to know the KH of both your tap water and RO water)- usually it works out to around 3 parts of RO to 1 part tap water, but your results will vary, so do your homework- lots of resources out there. And of course, there are other influences on the hardness and pH of your tank water, such as the minerals contained in substrates and rocks, which can impart some buffering capacity to the water in the aquarium, potentially "neutralizing" or at least impacting the influence of the acid-producing materials on the aquatic environment. 


Remember, the fact that leaves and other botanicals can release compounds capable of lowering the pH doesn't mean, if you add them to a hard water system, with a KH of 20 and a pH around 8.3, that you're suddenly going to achieve "Orinoco-like" conditions of soft water and low pH! Rather, with a lot of buffering capacity in the water, the impact of the botanicals on the pH may be surprisingly limited.


Where you can potentially run into trouble is in a system that already has soft, acidic water, and you add a large quantity of leaves and botanicals into the system at once. The rapid release of large amounts of tannins and humic substances can, indeed drop the pH far more significantly and rapidly than you'd want, creating an extremely stressful situation for the tank's inhabitants. (Not to mention, the influx of a significant amount of organic material to the aquarium, and its potential burden to the biological filtration, as well)


What happens when the pH plummets rapidly to levels well below, say 6.0? Well, a whole lot, actually! "Acidosis" can occur in fishes, resulting in potential damage to gills, skin, and internal organs. It gets worse: In highly acidic water, nitrifying bacteria (you know, the guys who help convert toxic ammonia into relatively harmless nitrate) essentially can "shut down", meaning you'd be totally dependent upon materials like zeolites and other chemical absorption media to remove toxic ammonia from your aquarium water. 

Although it’s possible to maintain an aquarium in this fashion, this requires a LOT of monitoring and no real margin for error on your part, so the reality is, that in most circumstances, you simply don't want to keep your aquarium's pH below 6.0...or even, say, 6.2. Just too many potential issues for some people to want to deal with, in my humble opinion.

For most hobbyists with water of moderate hardness, as stated above, the impact of an influx of leaves botanicals in an otherwise stable system will be negligible: Perhaps a slight drop in pH over time, and a definite aesthetic influence (i.e.; the color of the water!). However, just because the water is "brown" it does not indicate that it's soft and acidic, of course! 


 Like so many things in aquarium keeping, the extent to which we put our animals lives at risk is in our hands. Using leaves and botanicals to help foster "blackwater" conditions in an aquarium is no more dangerous than any other aspect of fish care. It's not all doom and gloom..You simply need to be aware of the potential of these materials to impact the aquatic environment, just like anything you add- from substrate to rocks, to driftwood-in any type of aquarium. 


The need to monitor a blackwater system carefully is different than  say, running an African Rift Lake cichlid tank or a reef aquarium- you simply need to be aware of what's going on in your water…it’s part of the game. Blackwater aquariums, like many other "specialty" aquaria, are not "set and forget" systems. They require monitoring, management, and observation on a continuous basis.

The idea of “pre-conditioning” your source water with materials such as Catappa leaves, etc., before it's utilized in your aquarium, makes a lot of sense. You can "mess with it" as much as you need to in remote containers to get it exactly how you want it, so you know going in exactly the parameters of the water being added to your display. From there, you monitor the impact of your water on the existing aquarium environment. It's perhaps a slower process, but in terms of control of water parameters, is ideal. 



Remember, a surprisingly large number of fish species that hail from soft water environments, such as Tetras, Apistogramma, Gouramis, Killies, etc, can adapt to harder, more alkaline water. Some may not look as spectacular as they do in soft, acid waters, and may not breed as easily, but the fact is that you don't have to obsess over creating these conditions in your aquarium to keep many of these fishes successfully. 






Many are produced commercially in conditions significantly different from those found in their will environments. Perhaps you simply want to run a "hard-water/tinted" system, which has the aesthetics of a real blackwater environment, and far less potential issues...that's fine and a good compromise- but you still need to be aware of what's going on in your water like in any other aquarium.

Just having stability of pH/hardness in your system, plus a little influence of the natural tannins and other compounds found in aquatic botanicals, can give your fishes some of the positive effects of blackwater environments, at least from an aesthetic standpoint, without significant risk associated with actively managing potentially unstable water chemistry.



Bring on the blackwater!

Okay, let’s say that I haven’t scared you off with all of that talk about the potential problems associated with using leaves and other botanical materials in your  aquarium! How would you then utilize these leaves in your aquarium? If you’re striving for an aesthetic component, and want to replicate the conditions of blackwater environments, these leaves are perfect! Blackwater streams and rivers are fascinating subjects for recreating in our aquaria!


A blackwater stream or river flows through forested swamps, wetlands, and flooded fields. As leaves fall from overhanging trees into the water and decay, the tannins bound up in these materials are released into the water, making it transparent, acidic, and darkly stained, looking for all the world like coffee or tea! The breakdown of Catappa leaves in closed aquarium systems mirrors what happens in nature, and offers many possible benefits for fishes that come from blackwater environments.




As mentioned previously, blackwater rivers and streams have different chemical composition from "whitewater" environments, which has lead to the formation of flora and fauna that differs significantly from what are found in other types of waters. In general, blackwater rivers have large numbers of organisms like rotifers, but fewer organisms like crustaceans and mites. You won't find snails to any great extent in blackwater systems, because it is difficult for them to build their shells in these calcium-poor environments. Sodium, magnesium, potassium and calcium are found in much lower concentrations in blackwater systems than in other types of aquatic systems, and with minimal amounts of dissolved ions, the water has much lower conductivity than you'd see in a "whitewater" system as well. Blackwater rivers, like the Rio Negro, are incredibly high in fish biodiversity, and it's estimated that they are home to over 700 known species, with around 100 being endemic to this river environment!


The leaf litter, in particular is a dynamic and fascinating component of the blackwater environment, one which would be fascinating to replicate in the aquarium!


In wild habitats, there may be literally hundreds of fishes per square foot inhabiting the matrix of botanical materials on the bottom of stream beds, which consists primarily of leaf litter.  As dead leaves are broken down by bacterial and fungal action, they develop biofilms and associated populations of microorganisms ("infusoria", etc.) that are an ideal food source for larval fishes. When you take into account that blackwater environments typically have relatively small populations of planktonic organisms that fish can consume, it makes sense that the productive leaf litter zones are so attractive to fishes!


The potential health benefits for aquarium fishes residing in carefully-controlled "blackwater" conditions are numerous, ranging from greater disease resistance to increased spawning activity, and, as documented in several studies, higher-yielding hatches with less incidence of fungal outbreaks in egg clutches. Some animals, such as Plecostomus catfishes, characins, and even ornamental shrimp, derive supplemental nutrition from grazing on these materials.


Obviously, as leaves break down, the look of your aquascape will "evolve", literally "morphing" into a different scene over time. Consider leaf litter a "transitional" or "consumable" product that needs replacing over time. The aesthetic, though alluring, isn’t for everyone!


One of the most common questions I’m asked is how often you need to replace your leaves. Here's my take on the subject:

 You don't have to remove the decomposing leaf litter, unless you simply don't like the look. Part of the "charm", if you will, of leaf litter is that it is a surprisingly dynamic environment to watch. However, there are situations where leaving the material in could be detrimental: If it's creating unsanitary conditions, such as trapping excessive amounts of uneaten food, fish waste, etc, and degrading water quality- then you'd want to a)review your husbandry practices and b)consider siphoning some of the material out. For most of us, however, I'd say to just leave the stuff in, and add new leaves as needed to maintain the "density" and aesthetic you want. 



Leaf litter zones comprise one of the richest and most diverse biotopes in the tropical aquatic ecosystem, yet they are seldom replicated in the aquarium. I think this has been due, in large part- to the lack of continuous availability of products for the hobbyist to work with, and just maybe, a “mindset” among aquarists that a “natural” aquarium must look “pristine” and almost “sterile” to be considered “healthy.”


The dynamic of life in this unique environmental niche can, in my opinion, best be summarized in this interesting excerpt from an academic paper on Blackwater leaf-litter communities by biologist Peter Alan Henderson, that is useful for those of us attempting to replicate these communities in our aquaria:


" within the litter is not a crowded, chaotic scramble for space and food. Each species occupies a sub-region defined by physical variables such as flow and oxygen content, water depth, litter depth and particle size…this subtle subdivision of space is the key to understanding the maintenance of diversity. While subdivision of time is also evident with, for example, gymnotids hunting by night and cichlids hunting by day, this is only possible when each species has its space within which to hide.”


In other words, different species inhabit different sections of the leaf litter, and we should consider this when planning, creating and stocking our leaf littler biotope aquariums.


How would you construct a leaf litter zone in your tank? 


As hinted at in the excerpt above, you should use a variety of materials, ranging from various leaves to seed pods, to driftwood.

In the aquarium, consider both practicality AND aesthetics when replicating this biotope. Much like in nature, you also want a diverse mixture of botanical materials, some which may decompose rather quickly (such as Catappa leaves and Guava Leaves), and others which have "duration" and last much, much longer (like Loquat leaves, Oak leaves, seed pods, Catappa bark, driftwood, etc.).


Regardless of how you choose to employ Catappa leaves and other botanicals in your quest for a "blackwater" aquarium, it's important to take a cautious, responsible, well-informed approach, and leave nothing about you water parameters to guesswork. Knowledge is a key to success and stability in all areas of the aquarium hobby, and you should arm yourself with as much information on your aquariums as possible, for the benefit of all of the animals under your care.



Chitmanat, C., Tongdonmuan, K., Khanom, P., Pachontis, P. and Nunsong, W. (2005). Antiparasitic, Antibacterial, and Antifungal activities Derived from a Terminalia catappa solution against some Tilapia (Oreochromis niloticus) Pathogens. Acta Hortic. 678, 179-182

DOI: 10.17660/ActaHortic.2005.678.25


P. A. Henderson and I. Walker (1986). On the leaf litter community of the Amazonian blackwater stream Tarumazinho. Journal of Tropical Ecology, 2, pp 1-16. doi:10.1017/S0266467400000547. 


I Walker, P.A. Henderson- Journal of Fish Biology JFIBA 9, 1990 Spatial Organization and Population Density of the Fish Community of the Litter Banks within a Central Amazonian Blackwater Stream


Hales, J., and P. Petry (2013). Rio Negro. Freshwater Ecoregions of the World. 

Scott Fellman
Scott Fellman


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