Shane's World Right Catfishology Right DIY Blackwater: Make your Own Oak Leaf Extract

Article © Deborah Claro Childress, uploaded July 03, 2009.

Most aquarists are familiar with the terms white water and blackwater, which are used to describe two types of water that many aquarium fishes originate from. White waters are typically distinguished by their higher sediment load, neutral to alkaline pH, having some measurable hardness, higher oxygen levels, and typically a constant, and often strong, current. White waters take their name from the coloration of their water which can range from nearly clear to the color of coffee with cream. Blackwaters are the opposite of white waters in every way. They are characterized by a very low sediment load, acidic pH, having little to no measurable hardness, lower oxygen levels, and a slack current if any. The color of the water can range from that of weak tea to strong black coffee. Blackwaters are created when a watershed with a poor mineral content, typically originating in older mountain ranges that have been leached of minerals or large swamp plains, flows through a heavily vegetated area. Because of their slow currents, decaying vegetation stays in contact with the water over time and the tannins they release in the process both stain the water and further lower the pH.
Some typical blackwaters where aquarium fishes originate include:

  • South America: Amazon basin; Rio Negro, Rio Apaporis, Rio Coari, Rio Tefe, and the Vaupes.
  • Orinoco Basin; Atabapo, Caroni, Inirida, Ventuari, and Vichada.
  • Indonesia: Sabangau River

In part due to their acidity, blackwaters have very low bacterial and fungal counts. Common parasites and their vectors, such as snails and mites, are also mostly absent due to the low mineral levels. For these reasons many blackwater fishes are considered “touchy” or difficult to maintain. Truth be told, very few aquarists attempt to replicate the exact parameters of real blackwaters as these conditions, typically with a pH of 4-5 and no measureable hardness, are very unstable in a closed aquarium system. What we can shoot for, however, is a reasonable facsimile of a fish’s natural environment although constrained by the fact that we are trying to replicate a complex system in a small glass box. Aesthetics also play a role since, for example, higher plants are usually absent or very rare in blackwater environments.

Aquarists have known for years that adding tannins to their aquarium has several beneficial effects and thus, a number of commercial blackwater tonics are on the market. The traditional blackwater tank typically includes filtration over peat, which both softens the water and leaches tannins that color the water. Deborah’s article, below, offers an alternative that can be dosed as desired to create blackwater-like conditions for aquarists that live in areas where oak trees occur. Even in light doses the addition of tannins promotes the health of the fishes, helps bring out their best colors, and sometimes promotes spawning activity.

Blackwater: Make your Own Oak Leaf Extract
Almost everyone who keeps aquarium fish has heard the term "blackwater" and knows this refers to the tannin-stained waters of some of the rivers, streams, ponds, and creeks of the world, most of which are found in North America, South America and Southeast Asia. Blackwaters are acidic and soft, often still or with little current, and with temperatures on the warmer side. They are home to many popular species of aquarium fish. Oak leaf extract mimics some aspects of the blackwater environment, without requiring you to layer the bottom of your tank with decomposing organic debris and decaying plant matter. Oak leaves are very popular for creating the leaf litter that so many aquarium fish seem to enjoy, and they are readily available in many areas of the world.

If you want to try it for yourself, here's the way to make about 1.5 liters (about 1.6 quarts) of oak leaf extract:

1. Collect some oak leaves.
They must be clean and dry before you use them, but they can be wet when you collect them. The collection area is important. It needs to be in a place where the leaves are not exposed to pesticides, road tar, or other pollutants. This culvert and stream bed is in Henrico County, Virginia, USA. I've collected from here before - mostly lizard's tail (Saururus cernuus) and dead wood for use in the aquarium.
2. Choose about 15 to 20 large, intact leaves.
If they are wet, let them dry and then clean them. Wipe off any insect debris, cocoons, dry dirt, and remove any acorns. After drying in the sun, these attractive leaves are ready to be used.
Dry oak leaves
Dry oak leaves
3. Soak the leaves
Use about one and a half liters of water in a pot which can later be boiled. If you have an old pot, fine, but this procedure will not harm your regular cooking pots. I left my leaves to soak for 48 hours, but 24 would probably work, too. Let's say somewhere between 24 to 48 hours of soaking in plain, cold tap water. Note that I did not dechlorinate the water at this point.
4. Boil the leaves
This can be done using the same soaking water for about 15 minutes. Longer will not produce a darker brew, so I'd say 15 to 20 minutes is plenty. Here is what my "witches' cauldron" looks like after the boiling is over. The extract looks and smells like a dark brewed black tea. I don't suggest this to anyone else, but I tasted a small amount for the article. The extract has a slightly bitter, tannic acid taste. Let the liquid cool. You may let it sit in the pot for a few hours longer, before going on to step 5.
Boiled leaves
Boiled leaves
5. Filter the cooled extract through a paper filter.
I set up the bottle I was going to use with a funnel fitted with a paper coffee filter. You have to pour the liquid slowly, and change the filters when the trickle slows to a slow drip. I went through three paper filters to get 1.5 liters (.4 US gallons, 1.6 quarts) of extract.
Filter and funnel
Filter and funnel

The extract can be stored in any ordinary clean bottle, plastic or glass. I'm using an empty drinking water bottle made of clear plastic, but I store it in a dark cabinet. To make greater quantities of the extract, simply increase all measurements proportionally. You'll need a very large pot, though! If you want a stronger concentration, add more leaves to the soaking water in step 3, and then follow all other steps as stated. Dry oak leaves may be stored almost indefinitely indoors, in a dry place.

How to add oak leaf extract
Add oak leaf extract to your aquarium by measuring the desired amount into a cup of aquarium water, and then pouring the mixture into the tank, slowly. Add dechlorinator to the mixture if a significant amount of extract has been used.

Lowering pH
Will oak leaf extract lower the pH of water? Yes, but large quantities of it may be needed to lower it significantly. You may want to test your extract's strength before using it in the aquarium. Experiments with small amounts of extract and 250 ml (about one cup) of plain water indicate that 25 ml of extract are needed to lower the pH from 7.6 to 7.0 in my local water. Results will of course vary based on the buffering capacity of your local tap water and even small amounts of extract added to rain water or reverse osmosis generated water may significantly lower the pH. You will need to do your own experiments and decide if oak leaf extract is right for your set-up. Oak leaf extract will stain the water anywhere from a very pale tinge of yellow, to a mild yellowish-brown, to the dark reddish brown of a very strong brewed black tea. It will also lower the pH somewhat - the hardness of your water will determine how low your pH will go easily. If your water is hard it will be difficult to get a really low pH, and unless your KH is on the low side, the pH will hold stable at the higher reading.

Water conditioning
My intention was to condition the water and to add some element of the natural environment that would be missing, by definition, from municipal tap water. A slight lowering of the pH was expected and welcome. I did not intend to alter my tap water significantly with this method, nor do I recommend doing so to anyone else. pH crashes could be the result.

Sample habitat
It is not necessary to duplicate every aspect of the natural environment to be successful. I have an awesome set-up with three species of fish from the true blackwaters of the Rio Negro and the Orinoco River basin in South America. The biotypically correct features of this system are warm temperatures of 78-80°F, one or two pieces of oak leaf litter, waterlogged wood, subdued lighting from skylights only, and a low current. Submersed plants are not typical in blackwater habitats, so I don't use many of them. I stick to one or two species and prefer slow-growing Asian varieties which tolerate my low-light conditions very well and flower above the water line. Buffering is low with a KH of 2, but water hardness is between 4-5 degrees, or 67-83 ppm. Not particularly hard, but not negligible, either. The pH is steady at 7.2.

75 gallon blackwater habitat
75 gallon blackwater habitat

A conditioned South American “blackwater” habitat may look something like this one. Here, collected wood, leaf litter, and sand are the backdrop for a number of slow-growing Asian plants, chosen for their growth habit and lighting requirements.

Back to Shane's World index.