Biotope aquariums (tanks modeled after natural ecosystems) are some of my favorite freshwater setups. Unlike the pink gravel and bubbling treasure chest desktop tank, a biotope makes you think of fast flowing jungle streams, rocky lake beds, or a placid, weed-choked tropical river.
Blackwater aquariums are a popular type of biotope aquarium that are a little more challenging than most. The water conditions are more exacting than most freshwater tanks. They are on par with marine aquarium systems in difficulty but just as richly rewarding once established.
Curious yet? Then let’s dive into the fascinating world of the blackwater aquarium.
What is a Blackwater Aquarium?
If you spend a fair amount of time outdoors you’ve likely come across a blackwater body of water before. They are found all over the world but are typically formed in enclosed areas like ponds and temporary pools. But in certain conditions where fresh water runs over mineral-poor soils into plant-choked forests, you can get flowing blackwater streams and rivers.
Blackwater rivers exist in places like Indonesia, Venezuela, Brazil, and other parts of the world where water runs through flooded forests, peat accumulations, dead wood, or swamps.
Most blackwaters are in tropical rainforests but not all. Many of the peat bogs in the United Kingdom and Canada are also blackwater environments, as are many swamps and rivers in the Southeastern United States.
These waters are stained dark through the accumulation of organic matter leached out from these dead plants. Plant tannins and humic substances slowly decay and buffer the water towards acidity or pH <7.0 (preventing the pH from easily becoming alkaline or >pH 7.0).
There are a number of popular and less well known aquarium fish that are found in these regions, including many tetras and cichlids. Setting up a blackwater biotope ensures that these fish have the best possible conditions for their health. Blackwater environments are often just the push these fish need to breed as well!
So if you want to learn more about this type of biotope aquarium then let’s break down how to set up and maintain a blackwater aquarium!
- Common Names: Blackwater biotope, Swamp biotope, Bog aquarium
- Typical Regions of the World: North America, South America, Southeast Asia, Australia
- Aquarium Size: Any
- Ease of Care: Moderately Difficult
Setting Up a Blackwater Aquarium
A blackwater tank is a little challenging to get up and running due to the specific water conditions needed. But once you understand what to add and what to prevent getting into the tank, you’re well on your way to success!
Aquarium Size
One of the most fun aspects to running a blackwater aquarium is that you can create one using any sized tank. In fact, a blackwater nano aquarium can actually be easier to maintain than a larger one. This is because of the exacting water conditions blackwater tanks require.
Since it’s best to start with ultrapure water and treat it with plant tannins, smaller aquariums are easier to keep treated and refilled. Many blackwater fish are especially small as well, including several nano fish. These fish are suitable even for aquariums as small as 5 gallons.
Of course, you can set up blackwater aquariums to be as large as you like. Tanks 55 gallons in size or larger are perfect for stately blackwater cichlids like Discus, Angelfish, and Uarus (Uaru amphiacanthoides).
You’ll also have space for schooling species like Silver Dollars and solitary large blackwater catfish like Hoplo cats (Megalechis thoracata). The only real difficulty is treating these larger aquariums when doing water changes but we’ll discuss this in a bit.
Water Conditions
When you see a blackwater aquarium for the first time the tea-colored water is striking and immediately makes you think of a natural pond, stream, or river. In order to maintain a successful blackwater tank we’ll need to add these substances ourselves.
There are two main types of plant tannin sources: slow-release and instant additions. We always want to have several slow release sources on hand as well as an instant blackwater mix we can add for water changes.
If you do a water change with the wrong kind of water, slow release sources may not easily bring your water conditions back to where you want them. The major sources of slow releasing humic substances are peat, driftwood, Indian Almond leaves, and other slowly decaying plant debris that’s naturally high in these chemicals.
Ideally, you’ll use a mixture of all three materials when setting up your tank for the first time. Mixing peat into the substrate is an excellent way to ensure your tank is buffered towards acidity for the long-term.
You’ll want to choose a substrate that’s either rich in organics itself or pH neutral. Acidic substrates include some clays, soil substrates (used in Walstad aquariums, and some aquarium planting substrates such as ADA Amazonia. You can mix peat into the substrate and then cap it with a ½-inch layer of sand to keep it from floating away.
pH neutral substrates include freshwater sand and gravel, as well as most but not all clay substrates. You don’t want to use marine sand or gravel, which is usually made of crushed aragonite and/or limestone. This material will dissolve in water and send your pH and hardness sky-high. We want this for African Cichlid and saltwater tanks but not blackwater aquariums.
Products for Adjusting Water Parameters in Blackwater Aquariums
- Tannin & Humic Acid sources: Peat, Driftwood, Bottled tannins, Indian Almond Leaves, decaying plant matter. These also act as acidity buffering agents
- General Hardness (GH) reducing resins
- Distilled or Reverse Osmosis (RO) water
Blackwater Conditioner
Let’s take some time to discuss the other most common sources of plant tannins. Having a bottle of blackwater extract is essential to aquarium maintenance. Anytime you need to do a water change you’ll need to quickly add enough tannins to rebalance the water chemistry.
Slow-release sources of plant tannins can’t usually match a sudden shift in parameters due to a water change. And even the largest piece of driftwood will eventually run out of humic substances to leach.
Many brands, such as the one above, don’t stain the water dark yet still contain the humic substances needed. This is the best brand for aquarists who want the plant tannin benefits without the dark water. But if you have other sources of tannins, especially driftwood and peat, you’ll see your water begin to darken over time.
Blackwater conditioners are highly concentrated; a single capful can dose several gallons at a time so use sparingly. You’ll rarely need to be super precise; we just need enough to dose newly added water and replace any tannins that have broken down due to biological activity.
These agents increase the bioavailability of nutrients for plants and have effects on blackwater fish that aren’t fully understood but must be experienced to be believed. Blackwater fish like characins (tetras), corydoras, and many cichlids will often become conditioned to spawn within a few weeks once you approximate their natural habitat.
Indian Almond Leaves
Indian Almond leaves are a popular source of humic substances that have become very easy to obtain lately. Betta keepers are finding just how helpful plant tannins are for fish health. Bettas aren’t exactly blackwater fish but they are found in acidic, tropical waters that are full of plant debris. They will also thrive in blackwater conditions.
Indian Almond leaves are simple to use: just scatter a few on the tank bottom. They may float at first for a few days; a quick boil or tearing up the leaves sometimes aids in sinking. Otherwise you’ll simply have to be patient.
The leaves are good for about a month until they need replacing. In a small aquarium (10 gallons and under) be very cautious with how many leaves you add because they are quite potent. You can also boil the leaves to create your own blackwater conditioner if you don’t like the look of leaf litter or want to be more precise in your dosing.
Lastly, if you’ve added a few and after a few weeks you see your water beginning to clear without having done a water change it’s likely your slow-release Indian Almond leaves are out of tannins.
Driftwood
Aquarium driftwood is another major source of plant tannins. Given how much volume wood has it’s pretty much essential for a blackwater aquarium. If you choose the right kind of wood you’ll have a slow release source that can last months to years at a time.
Not all driftwood works well for this purpose. Stay away from cholla wood and other pale, quickly decaying “wood.” Instead, we want large pieces of dark wood.
Some of the best wood for blackwater tanks are Malaysian Driftwood and Mopani wood. These types of driftwood are dense and typically sink immediately as well, making aquascaping that much easier.
Unfortunately, driftwood can be a little pricey as it’s sold by weight. I always recommend buying pet store driftwood though because wild pieces can create more problems than they are worth.
They may have pollution stored within, come from trees full of toxic substances, or may contain snails, leeches, parasites, and other hitchhikers. Unless you can identify your wild driftwood with certainty and then treat it properly (via boiling or extensive drying), it’s not worth the trouble.
Filtration in the Blackwater Aquarium
Setting up filtration for a blackwater aquarium is a little more challenging than in a typical tank. We want a high level of dissolved organic material in the water. But most aquarium filters are designed to pull out and neutralize these agents, which would ruin the blackwater effect.
You can use a typical power or canister filter in a blackwater aquarium. But remember to remove the activated carbon before turning it on. Activated carbon is a broad spectrum dissolved organic molecule absorber.
Activated carbon (charcoal) doesn’t select for what we want and don’t want. Plus, all of those plant tannins would quickly over load it and keep it from absorbing the actual problem compounds.
In a blackwater aquarium, plants and a substrate full of microbial life do a lot of the heavy lifting. If you don’t want to grow live plants or you have a high fish bioload, then regular water changes are the best way to keep up with the accumulation of nitrogenous waste (ammonia, nitrite, nitrate).
Instead of activated carbon we can use handfuls of peat in the power or canister filter’s media compartment. It may take some experimentation to figure out how much peat to add since it can vary in tannin richness. The size of your media compartment and filter flow will also determine how effective peat is.
Simply start with a small amount in the same media pouch that you’d use for activated carbon or any other loose filter media. Note the color change, monitor the water conditions for a few days, and add more or less peat as required.
Water Changes
We also need to spend time considering the source of our water when maintaining a blackwater aquarium. These bodies of water are not only rich in organics but are very low in dissolved minerals.
The general hardness (GH) and carbonate hardness (KH) of blackwater environments should be in the 2 to 5 degree range, which is extremely low in dissolved minerals.
The main problem is that most tap water tends to be fairly to highly alkaline and often mineralized. So we can’t use standard tap water when performing water changes without some treatment. Two common ways to treat it is by using reverse osmosis (RO) or distilled water.
Reverse osmosis is the process of forcing water through specialized membranes that permit the passage of water molecules but not other molecules. The result is ultrapure water as needed.
Some homes have RO units installed because the water tastes better, minerals don’t accumulate in the pipes, and soft water is nicer to bathe in and clean with. If you don’t have a home RO unit installed then you can use partial or full distilled water when refilling your blackwater aquarium.
You’ll want at least a 50% ratio of tap to distilled water to cut down on the hardness. But remember that any evaporation reduces the water level and increases the concentration of dissolved minerals as they are left behind.
If you don’t top off the tank to account for evaporation and then do water changes you’ll still see a slow creep upwards in GH and KH (and therefore pH). If you don’t have constant access to RO water a smaller blackwater aquarium may be easier to maintain long term since you can provide a steady quantity of distilled water easily and cheaply enough.
Some aquarists advocate using hardness absorbing resins but I don’t recommend these unless you have literally no other options. These resins do work well.
But the problem is that they replace calcium and magnesium ions with sodium from salt, which isn’t necessarily better. These waters are also supposed to be low in sodium. So using purified water is better for blackwater tanks.
Blackwater Aquarium Plants
Plants are another source of plant tannins. If you see decaying leaves or dead stems it’s a good idea to simply leave them in place rather than removing them. They may look a bit untidy but with Indian Almond leaves and driftwood in place they fit right into the blackwater biotope.
We need to choose our plants carefully because blackwater conditions can be challenging for some to grow. The dark waters inhibit light penetration, meaning most high light and carpeting plants won’t work. On the other hand, the darker water also inhibits algae growth, helping the tank stay tidier looking.
Many popular plants can be found in blackwater streams and rivers. These include Amazon Sword plants (Echinodorus sp.), Hygrophila plants (Hygrophila sp.), Pygmy Chain Swords (Helanthium tenellum), Java Moss (Vesicularia dubyana), Java Fern (Microsorum pteropus), Anubias, and Cryptocoryne.
Most of these plants are also tolerant of low light to moderate lighting conditions; exactly what you’d expect in the dark colored water of these tanks. Still, you will need to provide full-spectrum lighting for most of these plants to do well. Especially considering the light levels are already reduced.
Remember that not all aquarium lights are created equally. Aquarium plants need light that’s photosynthetically active radiation (PAR). Standard incandescent and fluorescent fixtures don’t put out nearly enough PAR to begin with and the problem is even worse in a blackwater tank.
Fortunately, carbon dioxide supplementation isn’t really needed in a blackwater aquarium. Most of these plants grow rather slowly to begin with.
Also, the slow decay of organic matter in the water column and substrate contributes to carbon dioxide. Fish respiration also provides the plants with enough CO2 in most cases.
Most of these plants are from equatorial regions and thrive in very warm conditions (temperatures of 78-86℉). Fortunately, most blackwater fish also prefer these conditions.
The majority of aquarium plants prefer cooler conditions, less acidity, and a bit more mineralization. That said, each of these plants also grows in more temperate conditions.
Blackwater aquariums also make fantastic Walstad tanks if you like live plants. One of the main benefits to running a Walstad aquarium is that you won’t need a filter since the lush plant growth consumes all of the fish waste. Once it’s fully established you may not even need to do water changes – simply top off the tank as it evaporates.
Walstad aquariums use regular potting soil as a substrate. And most of these have peat, sphagnum moss, worm castings, and other sources of partially decomposed organic matter.
Once you cap your substrate with a layer of sand or gravel you have a steady source of plant tannins as well as an ideal substrate for rich plant growth. Walstad tanks, especially when brand new, are often tea colored as a result.
Still, you’ll still need to use the right kind of water to avoid mineral accumulation (see below) over time. The soil substrate will eventually become exhausted within a year or three and need replacement.
Fish for a Blackwater Aquarium
The number of fish suitable for a blackwater aquarium is larger than you might expect! Many popular aquarium fish were originally discovered in blackwater environments. While most of them have acclimated to the more alkaline conditions found in home systems they still do much better in acidic conditions.
Some of the most common blackwater fish include Neon and Cardinal Tetras, Corydoras, Otocinclus, Discus, Angelfish, Blue Rams, Kuhli Loaches, and Cherry Barbs. All of these fish are difficult to impossible to breed in alkaline pH chemistry.
Some less common but still easy to find animals include Chili Rasboras (Boraras merah), Earth-eater Cichlids (Geophagus sp.), Apistogramma Dwarf Cichlids, and Bee Shrimp (Caridina cantonensis). Chocolate Gouramis, Sparkling Gouramis, and Bettas also thrive in low pH waters.
The characin (Tetra) family is one of the most successful groups of blackwater fish on earth. While Tetras are by far the most common members there are also some outsized (and toothy) fish available.
Pacus, Piranha, and Silver Dollars are all blackwater fish as well. Just keep in mind that these fish have specific dietary needs and grow very large. Silver Dollars and pacus are both strict vegetarians that will eat any aquarium plants as well as hard terrestrial vegetables like cucumbers and even nuts!
Silver Dollars are manageable but still large (6-10 inches) and need to be kept in schools. Pacus can grow nearly 3 feet long and weigh dozens of pounds.
And they are schooling fish so think carefully about whether you want one (or several) because it’s not very easy finding a home for an adult Pacu. And the infamous Piranha species are all medium to large in size, schooling, and carnivorous.
If your pH isn’t super acidic (pH 6.0-7.0) the list expands even further. Most aquarium fish will do just fine in slightly acidic to neutral water. Guppies and other Livebearers work nicely, as will Barbs, Cichlids, and just about anything else that doesn’t require alkaline conditions.
The only real challenge are invertebrates. Nearly all freshwater invertebrates have hard shells created from calcium carbonate and other minerals. Blackwater conditions tend to leach away at the minerals of their shells, killing them over time.
Only a few snails and shrimp are specifically adapted to these conditions. Malaysian Trumpet Snails and Bee Shrimp do well, as do Bamboo Shrimp (Atyopsis moluccensis).
Good Fish and Invertebrates for a Blackwater Aquarium:
- Most Tetras, especially Neon and Cardinal Tetras
- Silver Dollars and Pacus
- Discus, Angelfish, and Uarus
- Guppies and Endler’s Livebearers
- Bettas and Gouramis
- Blue Rams, Apistogramma, Kribensis, and other Dwarf Cichlids
- Chili Rasboras, Cherry Barbs, and many small Cyprinids
- Kuhli Loaches, Corydoras, Plecostomus, Otocinclus, and other soft water Catfish
- Bee Shrimp, Bamboo Shrimp, Malaysian Trumpet Snails, and other blackwater-specific invertebrates
Poor Fish and Invertebrates for a Blackwater Aquarium
- African Cichlids
- Danios, Platies, Swordtails, Mollies, and other neutral to alkaline pH fish
- Most invertebrates
Conclusion
Blackwater aquariums are unique, challenging, but in my experience, well worth the effort. The fish positively glow in color when placed in a dimly lit tank with dark water. They are also much more likely to breed and display other natural behaviors.
These aquariums do take more effort to balance the appropriate water conditions. But with the knowledge in this guide you have exactly what you need to succeed!
