A blackwater breakdown


The blackwater biotope is all the rage in hobby circles, but is there more to it than just staining the water? Chris Sergeant explores the dark and murky waters.

At face value, muted lighting over a biofilm-coated bogwood centrepiece, atop a sand-bed heavily littered with decaying leaves and botanicals, all creating the tinted water, looks to tick all the boxes for a ‘blackwater biotope’. But from a scientific perspective, it is technically a blackwater aquarium? 

A recent episode of The Tint podcast saw Scott Fellman of Tannin Aquatics questioning whether the term ‘blackwater’ is now used as a generic descriptor for tinted, tannin-stained water. Given the many websites perpetuating this idea, he certainly has a point. With the ever-increasing popularity of blackwater biotopes, it feels only fair that we know exactly what is going on from an ecological perspective.

What is it?

Blackwater refers to soft, acidic water with a deep brown colouration, and while the tannins from decaying leaf litter, fallen botanicals and sunken wood play their part in its formation, their role is secondary. Instead, it’s the geology of a landscape that plays the focal role in its creation. 

I am certainly no limnologist, so it helps to use an example for context — in this case, the Río Negro. As one of the major tributaries of the Amazon, this blackwater river drains from the Precambrian Guiana shield, which is characterised by large areas of white sands, or podzols. These highly acidic soils form in forested landscapes, high in quartz and with a sub-surface layer high in metal oxides, notably iron and aluminium, and humus, yet low levels of minerals like calcium, magnesium, potassium, and sodium. 

As the river drains through and flows over this substrate, high levels of organic acids, in the form of fulvic and humic substances, leach out from the soil and broken-down vegetation and dissolve within the surrounding water bodies, leaving the characteristically acidic (pH 3.0-5.5), ion-poor conditions, rich in dissolved organic carbon (DOC) associated with blackwater. In conjunction, the decaying riverbed debris releases a variety of organic and inorganic compounds, including various carbohydrates, phenolic compounds and tannins. While the tannic acids do contribute to the tea-stained aesthetic, it’s these humic and fulvic acids, along with the elevated DOC, that are the main source of colouration. 

These ion-imbalances have ecological implications, influencing who can thrive and who cannot survive in blackwater environments. Calcium, for example, is an important component when it comes to the growth and strengthening of a snail’s shell, or the development of a shrimp’s exoskeleton. When levels drop, or are naturally lacking, snail shells can erode, crack or discolour, while shrimp encounter issues when moulting, and so as a consequence, they are scarcely found within these deficient habitats. 

Similarly, studies have shown that the micro-fauna composition found within Amazonian basin blackwaters alters in comparison to nutrient-rich whitewater environments. When tested, the blackwater environment’s plankton communities were dominated by Rotifera, while supporting far fewer crustaceans like Cladocera (water fleas), or copepods from the orders Calanoida and Cyclopoida. On a microscopic level, even bacteria require a degree of salt in the water and so are found in much lower concentrations than in other Amazonian water bodies, and this lack of bacterial life sees the resultant breakdown of the organic substances revert to humic acids.

Such conditions as those in the Rio Negro should have a knock-on effect on the fish, with the highly acidic, ion-poor conditions liable to instigate fatal ionoregulatory failure in the majority of species. 

Ionoregulation is the regulation of the balance of ions within the body, with freshwater fish being hyperosmotic. That is, their blood concentration is saltier than their surrounding environment, with fish losing salt ions via their permeable gills through diffusion, and taking on water instead. 

To compensate, freshwater species urinate profusely, while taking up sodium (Na+), chlorides (Cl−) and other ions via active transport. In such nutrient poor-waters, with sodium ions in short supply, the issue becomes exacerbated and the fish are unable to replace the ions at the rate they are lost, causing their body fluids to become diluted. In fact, in his book ‘The Amazon - Limnology and landscape ecology of a mighty tropical river and its basin’, Harald Sioli proposes that the blackwaters of the Rio Negro are essentially like ‘slightly contaminated distilled water’. 

That’s the theory anyway. Yet, reports suggest that these blackwater environments support 8% of the world’s ichthyofauna, with high levels of endemism too.

A selection of readily available botanicals.

How do they live there? 

The secret may lie with one comparatively understudied parameter — dissolved organic carbon (DOC). Defined as the fraction of dissolved organic matter (DOM) that can pass through a 0.45µm filter, and that contains approximately 50% carbon by mass, DOC is comprised of a diverse array of compounds formed by decomposing organic matter, with the major components being humic and fulvic acids. 

DOCs differ based on their origins, and can have a wide range of molecular weights and chemical structures. Autochthonous DOCs form in situ, from the decomposition of algae, aquatic plant-material and other micro-organisms in the immediate environment, whereas the allochthonous (or terrigenous) variety derives from the degradation of land-based plant materials. As with many blackwater environments, those in the Rio Negro are comprised of the latter. 

In the water, DOCs are characterised by their affinity for metal ions and protons, as well as their ability to bind to biological membranes — such as fish gills.
By doing so, they can provide protection for the fish against both metal toxicity and the exertions on the ionoregulatory systems. As a general rule, the larger the DOC and the darker it is, the greater its ability to protect and influence, and the Rio Negro contains some of the largest and darkly-coloured DOCs of anywhere in the world. Because of this, it was proposed that the Rio Negro DOCs had uniquely protective properties that enabled the fish living in such low ion, low pH conditions to thrive.

Various studies have been undertaken to assess how certain Rio Negro species survive and investigate the strategies employed, which vary by species, particularly between certain characins and cichlids. Some characins, like the Neon tetra, Paracheirodon innesi, Cardinal tetra, Paracheirodon axelrodi, and Black widow tetra, Gymnocorymbus ternetzi, favour a high affinity, high-capacity ion transporter. 

In P. innesi, this enables the ion regulation and sodium ion uptake to continue at the same rate at pH 3.5 as it does at pH 7.0. In other words, they appear to be completely physiologically unaffected by the issues caused through pH extremities and limited ion availability. For other cichlid species, like the angelfish, Pterophyllum scalare, rather than maximise their ion uptake, their approach is focused on reducing the rates of diffusive ion loss and limiting the overall net loss at a low pH instead, and so they have adopted a low-affinity, low-capacity ion transporter system. However, this system is pH sensitive, and at a range of pH 4.0 or below, ionoregulatory failure still occurs and the fish will die. 

Trans-epithelial potential (TEP) in a fish is the voltage difference between the extracellular body fluids (e.g., blood plasma or interstitial fluid) and its external water surroundings. So, we know freshwater fish have saltier body fluids than their immediate habitat, and therefore salt ions diffuse out through their gills. We also know that the ion-poor waters of the Rio Negro would, in theory, exacerbate this ion loss further, and so it’s here that the benefits of the Rio Negro DOCs on a fish’s ionoregulation become apparent. 

The DOCs serve to lower the TEP value, so as well as preventing the inhibition of sodium ion uptake and ammonia excretion, the strategy favoured by the characins, they also protect against increasing diffusive sodium ion loss in fish exposed to acidic waters, benefiting the cichlid examples too. Not only that, but they also reduce the gill binding and toxicity effects of metals present too, and after chronic exposure to Rio Negro DOCs, these protective benefits to the fish persist, even in the absence of any further DOCs.

Cardinal tetras have evolved to blackwater conditions. 

No substitute

Numerous studies have been tried using synthetic water with a similar ionic composition, but fish kept in Rio Negro water always exhibit a better ionoregulatory performance. Research into the effects of acute low‐pH exposure on ion balance showed that fish collected directly from Rio Negro waters (Black piranha, Serrasalmus rhombeus, Banded leporinus, Leporinus fasciatus, and Pacu, Myleus sp.) exhibited far lower rates of Na+
and Cl− loss than an aquacultured species (Tambaqui, Colossoma macropomum) kept at the same acidic (pH 3.0) conditions. 

In fact, such is the protection afforded by these Rio Negro blackwater DOCs, it has been shown to mitigate these effects for
a non-native species too. A project led by Rafael Duarte from the National Institute for Amazonian Research, exposed Zebrafish, Danio rerio, to ion-poor, pH 4.0 water with DOCs isolated by reverse-osmosis from the São Gabriel da Cachoeira region of the upper Rio Negro added. Rather than display ionoregulatory failure, like those individuals housed without the DOCs, the fish incurred a reduced loss of Na+ and Cl− and a stimulation of Na+ uptake — an almost full protection.

Some species, such as the Sardinhas, Triportheus albus, which can be found across white, clear and blackwater environments, show phenotypic plasticity, meaning elements of their physiology change between individuals across these different environments. Blackwater individuals sampled were found to have additional homeostatic mechanisms, relating to a reduced cell permeability and ionic regulation, meaning gene expression is in turn determined by the type of water the individual inhabits.

Such environments are not solely restricted to the Amazon. Plenty of North American and South-East Asian streams, swamps, peat bogs and wetlands, and most Congo and Lower Guinean rivers that flow through rainforests, are classified as blackwater environments, and a wide array of aquarium species originate from such habitats: numerous cichlids, Loricariid catfish, gouramis, tetras, pencilfish, hatchetfish, snakeheads, rasbora and loaches. This abundance of options, in conjunction with the rising emphasis on natural-looking designs, means you can see why blackwater aquascapes are so prominent. Having established what blackwater actually constitutes, the next question for many is how do you go about recreating it?

Leaves should be soaked prior to use.

Domestic recreation

Firstly, is your water supply soft or hard? Too soft and the pH-lowering effects of the décor may become exacerbated, whilst water that’s too hard may cancel out the botanicals’ effects on pH, so testing your water for pH, general hardness (GH) and carbonate hardness (KH) is advised. 

For a generic igapó-style set-up, seeking to replicate a seasonally-flooded forest floor, the next point is to simulate the water chemistry of a blackwater habitat, which means a low mineral, low bacterial count water. Unfortunately, for those living in particularly hard-water areas, using tapwater alone won’t cut it, so reverse osmosis (RO) water is your best bet. You’ll need to buffer the RO prior to use, either through commercial additives or through the addition of small quantities of tapwater. 

It’s important to remember that it’s not that these seemingly acidophilic fish enjoy the low pH conditions, it’s that the underlying water chemistry associated with blackwater regions enables them to survive. Seeing as nobody has commercially bottled Rio Negro DOCs for aquarium use, cranking the pH down to 4.0 is not the goal; instead aim for around pH 6.0-6.5, depending on your intended inhabitants’ requirements.

It’s easy to neglect the substrate composition and just focus on the botanicals, but to do so would be missing a trick, given that substrate acts as an important repository for decomposing organic matter. Given that substrates can vary between sand and silt through to mud and mulm, your options are wide open. 

One idea is to incorporate the ‘dirted’ approach, utilising a planting substrate to mimic this organic layer. Organic potting soil, rinsed and dried multiple times, works well as a mulm substitute, with a thin top layer of pre-washed sand, mixed together with ‘Fundo tropical natural substrate enhancement media’ or ‘Substrato Fino coconut substrate enhancement media’ sold by Blackwater UK and Tannin Aquatics. 

In with the botanicals

Next, you’re ready for the main focus of your hardscape: the dense leaf litter, the key characteristic of any igapó design. If you are remaining true to a biotope, Blackwater UK list their leaves by provenance, but otherwise, catappa, guava and magnolia will all work. A more cost-effective measure is to collect your own, with autumn being the perfect time to pick up newly dropped oak, sycamore, hazel or hornbeam leaves, as well as alder cones, to name but a few. Be sure to collect them away from roads and railways and dry them out as soon as possible first. Not only will this vegetation breakdown and decomposition contribute to the organic acid release, but the interstitial gaps provide a place to hide and hunt. 

A variety of other botanicals in the guise of seed pods, twigs, bark and palm stems all help to add texture, and in the case of larger pterygota or theluba pods and monkey pots, a perfect, partially enclosed spawning site too. For sheer quantity and variation, Crowders Aquatics would be my recommendation — just remember to boil and soak any additions first to help forgo buoyancy issues. You can collect, strain and bottle up the resultant ‘tea’ to use in your tank later on.

In terms of larger materials, my own preference is bogwood to recreate the fallen branch effect,
or pieces of manzanita orientated down from the surface to give the impression of a tangled network of roots overhead, but it depends on the exact layout you have in mind. Pre-soaking is a must, unless you want floating décor. Palm fronds make an eye-catching backdrop, but these rot quickly and will need replacing after a few weeks. Aquatic plants can be sparse, and would definitely need to have low lighting tolerance. Anubias, Bucephalandra or Cryptocoryne might survive in blackwater conditions. Floating plants, like Amazonian frogbit, will help diffuse light even further.

Filtration with high surface area media and low flow will help imitate the quieter conditions of the flooded forest floor, and for the most part, removing any activated carbon from the filter will prevent unwanted removal of the tannic and humic acids. If you are concerned about metal toxicity, or if your aquarium is becoming too dark, you could run activated carbon for a week or so each month as a compromise. With tank lighting, your aim is to recreate the dappled effect as the sun attempts to penetrate the forest canopy above, so use low level illumination and position the beam in a way so to accentuate the shadows. 

A naturally occuring podzol.


Even a simple 20% weekly water change has the potential to disrupt or raise the pH and dilute the colouration, so testing your water parameters, both current tank water and that to be added, is important. One trick is to prepare your water a couple of days in advance, either by adding old leaves, or a handful of pre-boiled, then cooled, botanicals, as this helps increase the acidity, thus buffering against the impact of a pH swing. Stability is crucial in blackwater habitats.

Fungal blooms on wood are common, and usually clear up without any intervention after a few weeks, but one you thing that may take a little getting used to is the ensuing biofilm. This is the ‘gunk’ that appears over the botanicals, essentially bacterial mats held together by a sugary matrix.
While perhaps not the most aesthetically pleasing, it’s all part of nature and they are present in almost all aquariums. If they do become too unsightly, you can remove the offending botanicals, give them a quick scrub and rinse and then replace them, but I prefer to think of it as a free food source. Numerous fish and shrimps will graze on the biofilm and the micro-organisms trapped or living within it, and providing your tank is sensibly stocked and well-managed, the blooms will subside in time. 

Having long been associated with anti-fungal and healing properties, blackwater and its associated organic acids also help enhance colouration and enable higher, healthier yields of offspring too. 

As Fellman says, ‘decomposing leaves, seed pods, and tree branches make up the substrate for a complex web of life which helps the fishes that we’re so fascinated by flourish.’ 

Fellman puts it perfectly when he suggests that if we want to shift our fishkeeping goals to a more ‘natural-looking, natural-functioning aquarium’, then the detritus and decomposing leaves, the tinted water and biofilms, could be seen as the ‘cost of admission’ in order
to do so.