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Blackwater: Basics and FAQ

  • Author MacZ
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Blackwater basics & FAQ

1. What is blackwater?

Blackwater denotes a certain water chemistry that can be found regularly in tropical, subtropical and sometimes moderate climates.

It is defined by low ion concentrations, low conductivity and low hardness, primarily. This makes blackwater a subset of softwater, which also includes so-called clearwater. Secondary defining factors are a low pH and high concentrations of humic substances.

Humic substances (colloquially often called “tannins”) are weak organic acids and compounds that add acidity and often also tint the water. Among these substances are humic acid, tannic acid, fulvic acid and dozens of other compounds.

Only water that meets these defining factors is “true blackwater”. Aquarium water that is tinted by humic substances but otherwise is identical to tap, can be referred to as “pseudo” or “faux blackwater”. This article is solely about true blackwater.

A Nannostomus eques male guarding his territory.

2. What are typical parameters for blackwater?

These are the typical ranges for blackwater parameters:

GH (general hardness, Ca/Mg): 0 – 1°GH

KH (carbonate hardness, e.g. CaCO3): 0°KH

TDS (total dissolved solids): 0 – 50mg/l (1 mg/l = 1 ppm)

EC (electric conductivity): 0 – 75µSi/cm

pH: 3 – 6

Note: The higher values for pH, TDS and EC overlap with clearwater parameters.

A faux blackwater tank

3. What are the advantages of blackwater?

Besides giving your fish the parameters they are evolutionarily build for, low hardness/low pH water has very low levels of bacteria that can either affect the fish directly or their spawn. True blackwater fish will generally be more healthy and stable.

A male Dicrossus filamentosus looking for food on botanicals.

If you can live with biofilms but don’t like to see algae in your tank, the tint prevents most algae completely below the upper 10-15cm.

4. Are there any downsides?

Pretreating water with an RO unit can, depending on its efficiency, use quite large amounts of water with RO to wastewater ratios of up to 1:5! So if you live in areas with regular droughts, high water hardness and/or high prices on tapwater you might want to overthink this.

You will also need a steady supply of botanicals or peat.

Dicrossus are very typical blackwater fish from the Rio Negro and Orinoco drainages.

5. What about plants in blackwater?

If you want a lushly planted tank, the reduction of light in the lower regions of the water column by the tint of the water will keep you from keeping plants with a high light demand. Also the fact that RO water contains virtually no nutrients likely makes adding a full spectrum fertilizer pretty much a given if you want to keep any plants alive.

In blackwater it’s best to keep plants that “cheat” to avoid the problems of blackwater altogether. So fully emersed plants like Pothos or Monstera are good choices, as are floating plants like Limnobium or Salvinia. Semi-Aquatic plants like Hydrocotyle or half emersed grown Echinodorus are also possible. When it comes to plants that are planted in the substrate, Nymphaea is able to cheat the conditions and grow floating leaves that gather light and CO2. Anubias and Bucephalandra can be placed near the surface to get enough light. They can also grow out of the water in that situation.

The emerse part of a blackwater riparium.

If you decide for a plantless blackwater setup, 50% waterchange a week are an absolute must, with no exceptions and barely any wiggle room.

6. What are common techniques to make blackwater in the home aquarium.

Note: The most important prerequisite to have is a STEADY SOURCE of very soft water.
Rainwater, RO (reverse osmosis), DI (de-ionized) or distilled water are a must-have for the following techniques.

a. Driftwood, botanicals and leaf litter

Likely the cheapest technique. You can either add the plant material directly to the tank/water or make extract from it.

Important: Botanical material will very soon start to decompose underwater and biofilms and bacteria will start to colonize it soon. The colonization process takes up a lot of oxygen, so add these materials slowly to a tank with livestock or wait some days before adding any fish.

Due to botanicals literally rotting and releasing small amounts of ammonia and other nitrogen compounds they count as bioload. In comparison to a clearwater or tapwater run tank the maximum stocking density is reduced by 25-50%!

With botanicals KH has to be below 1°KH to change water parameters significantly. It takes months to lower pH with this technique, but in the end it stabilizes between 4 and 6. Additionally botanical mulm and leaf litter beds are important for the biological balance in such a tank.

Visible humic substances are reduced with water changes, so if you want the water tinted darkly, go for extracts of alder cones or rooibos tea you can add to adjust the tint.

A variety of botanicals including IALs, Alder cones, palm leaves and oak leaves.

b. Peat

We’re not talking peat moss (genus Sphagnum) here, but actual peat. Peat is several steps of decomposition away from peat moss. It’s concentrated humic substances.

Peat is most often added in a filter media bag, either just hung in the tank or added to the filter.

For this technique the water can have a KH of up to 5°KH to work. The higher the KH, the more sense it makes to pretreat the water. Otherwise the pH and KH may rise slightly with every waterchange, depleting the peat over time making regular replacement necessary.

As with botanicals the humic substances in peat also stabilize the pH between 4 and 6.

A blackwater tank facilitating peat. Note the much darker tint than in an exclusively botanical blackwater tank. At the time of taking the picture there there was spawning activity of Cleithracara maronii, Corydoras paleatus, Gymnocorymbus ternetzi and Hyphessobrycon herbertaxelrodi, with the latter having numerous juveniles swimming around at half adult size.

c. Acids

As humic substances are only weak acids that neutralize carbonates (KH) before lowering pH, one can technically use stronger acids like hydrochloric acid or sulphuric acid. Commercially available pH-Down products make use of this and are hence only to use with extreme caution.

Downside of this technique is, that without adding humic substances the pH is not stabilized, not even in the low range. So unless you have to reach extremely low pH and KH (e.g. for licorice gourami) I advise against it. Especially for blackwater beginners.

Note: This technique requires routinely handling potentially dangerous chemicals and dosing errors can cause quick fish deaths!

ONLY use this technique for pretreating the water!
ALWAYS use protective measures for your own safety!
NEVER add these chemicals directly to your tank!

7. Is blackwater absolutely necessary for my fish?

In short: No. Soft water fish are able to acclimate to higher hardness and pH. So it is definitely possible to keep these fish in tap water. The downside is the strain harder water puts on their bodies, especially inner organs like the kidneys. Soft water fish in hard water tend to be more sickly and die prematurely of organ failure. Still they can live in harder water for years without visible problems.

But seriously: It depends. Many aquarium fish come from very soft water and often blackwater in the wild. For wild caught specimens setting up true blackwater may very well be advised and many of these species, even if farmed, tank raised individuals, will only really shine in blackwater. Some species, although tankbred in harder water, will still be more stable and healthy in it as well.

If you are looking to breed blackwater species, though, you are dependent on reproducing their natural habitat’s water parameters.

Cleithracara maronii in a blackwater tank.

8. They say the nitrogen cycle doesn’t work in pH below 6 – 6.5. Is this true and how can a blackwater aquarium then be cycled?

The nitrogen cycle is, in contrast to what manufacturers of bacteria products say, still working in low pH. BUT the microorganisms that are responsible for metabolizing Ammonia to Nitrite and Nitrites to Nitrates are not the same that you can buy in bottles. Some of the low pH microorganisms skip the Nitrite stage and put out Nitrate as their final product. So no worries if no Nitrites appear ever.

mulm 2.jpg
Accumulated mulm in a blackwater tank. This is the stuff that makes most blackwater tanks even possible.

In low pH different species and completely different groups take over the cycle. Instead of Nitrosomonas and Nitrobacter you find Nitrospira bacteria and several kinds of archaeans taking over the cycle. They work slower, but steady. They also take longer to grow. Fully cycling a blackwater tank can take months! This also influences stocking density. (see above)

As in blackwater the pH is usually significantly below 6.8, Ammonia is present not as Ammonia (NH3) but as Ammonium (NH4), which is barely as toxic. Typical test kits for Ammonia also measure Ammonium. Readings of up to 2.0 mg/l of Ammonium are ok, still a water change is advised!
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I learn new things each time I read, and this is at least my 4th read. Great, informative article, MacZ!
Excellent! Very informative and easy to read. The photos are great and illustrate perfectly what the differences are.
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Did a great job informing on black water. I fully understand the concept now.

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