Blackwater Aquarium: An Ultimate Guide

Blackwater aquariums have a certain mystique. The tannin-laden water, silty bottoms, and general wildness of the tank can make an amazing display, and blackwater environs are actually very natural for many of our favorite fish. Setting one up is a challenge, but it’s one that most aquarists will be able to handle readily.

So, let’s dive in and I’ll answer your questions and show you how to set up a blackwater aquarium with minimal fuss!

blackwater tank

What is a Blackwater Aquarium?

Blackwater aquariums are meant to mimic the environmental conditions of certain tributaries and rivers in the Amazon. Blackwater environments exist in other places, but the term is almost always used to refer to a type of Amazon biotope set up at home.

Blackwater is… well, it’s basically tea. The same compounds that color tea when the leaves are steeped, called tannins, color the water in these areas. This lends blackwater the orange-brown coloration that infuses the water and also changes the chemical properties.

This high inclusion of tannins and related compounds creates a very acidic environment. PH readings as low as 3.8 have been found in these rivers, but most home aquaria will end up being a bit higher.

This acidic environment also dissolves plant life, creating slightly thicker water than you’ll see in most freshwater aquaria due to the particle inclusions.

It’s a different look and a different experience, even if you keep the same types of fish.

Blackwater conditions are very different than those in a normal freshwater aquarium, so fauna and flora choices will have to work around the water chemistry.

In a blackwater aquarium, we generally use rotting plant material to create a low PH, tannin-rich environment. There are a few ways to do it. Most often, we alter the substrate to create the effect and change the way we use filters to avoid stripping the tannin out of the water.

At its core, a blackwater aquarium is one that mimics natural blackwater environments by using plant material to increase tannin levels and decrease the PH of the water.

Other Blackwater Biotopes

While the most widely known blackwater environments are shallow rivers in the Amazon, blackwater tanks actually represent a much larger set of conditions. There are examples of blackwater areas on every continent except for Antarctica.

These range from shallow tributaries to peat bogs.

Many people choose to create an actual biotope when putting together a blackwater tank. This can be a huge influence on the plants, animals, and invertebrates that are included.

One of the best ways to learn about blackwater tanks is to study individual biomes. The ebb and flow of nature can help us create the best environments for our animals, and creating balanced biotopes is a true challenge.

I’ll be focusing on Amazon-type biotopes for the purposes of this article, but the basics are the same no matter which country your desired fauna come from.

Further Clarification

It’s possible to maintain a neutral, or even alkaline, pH in a tank that’s loaded with tannins. It’s actually fairly common, and it causes a lot of confusion for those who are looking for hard data.

I’ll be talking about tannin-rich tanks underneath the 6.0 pH range unless otherwise noted for the rest of this guide.

In alkaline or neutral systems, the main change is that the darkened water will absorb some of your tank’s light.

How to Set Up a Blackwater Aquarium

A blackwater aquarium needs to be set up properly from the very beginning. They take a little bit more care than setting up a regular aquarium, but they’re generally easier to manage than marine tanks.

What You Need

You’ll need some extra stuff to set up a blackwater tank.

  • Tank- Any size will do, I’ve run them as small as 5 gallons. I recommend 20 for a beginner to keep things more stable, but tank size isn’t a limiting factor here.
  • Filters- Any filter appropriate for the tank size. There’s some level of controversy about using or not using activated carbon which we’ll cover below.
  • Heater- Standard for the size of the tank.
  • Test Kit- Non-negotiable. Get one, you’ll need it.
  • Substrate- Fine and sandy is ideal for these kinds of tanks, especially since there will be leaf litter on the bottom.
  • Lighting- Standard LED lights for your size of tank are great, you may need to step it up a bit depending on your plants.
  • Driftwood(Optional)- Driftwood can provide extra tannins and it’s a natural part of blackwater environments. You don’t need it, but I recommend throwing some chunks in there for decoration and further water stability.

But that’s just the usual. You also need the right stuff to create the blackwater environment. As a general rule, the easiest way to do this is to add commercial mixes of dried leaves. These leaves are devoid of most nutrients, so their main contribution to the tank is just tannins and acids to lower the pH.

Try to use RO or distilled water for blackwater tanks. The neutral pH and lower mineral content make it a lot easier for the tank to reach the right conditions.

Indian almond leaves are among the most popular used. They’re often used in small amounts in Betta tanks as well to create a healthier environment for the fish. Another common additive is alder cones.

On a personal level, I used local leaf litter from under red oak trees. Fallen red oak leaves are safe for the aquarium and I had them in abundance, but I would usually boil them for 5-10 minutes before adding them to the tank to avoid introducing pathogens.

Don’t just grab random leaves and start sticking them in there. A lot of leaves are poisonous or can cause major problems. If you’re not sure… stick with Indian Almond Leaves.

1. Set Up the Tank Normally

Go through the usual steps. Place down your substrate, make sure your equipment is running and don’t add fauna just yet.

Blackwater-safe plants can be added at this stage, but you may want to wait for some plants. The rapid change of conditions can cause problems for plants.

It may not kill them, but it’s not a lot of fun to wait for them to grow back after the pH swings downward.

2. Add Your Plant Matter

Dosing the plants for a blackwater tank is… inexact at best. You’re unlikely to harm the tank just by adding too many leaves but you definitely need to strike a balance between visibility and water chemistry.

For starters, 2-3 Indian Almond Leaves per 10 gallons of water works well. Alder cones seem to work best at 1 per 5 gallons of water. With the red oak leaves I favor, I just throw in a cleaned handful per 15 gallons.

Most stuff floats for a day or two before dropping into the tank.

The big thing here is just not to stress about it. You can modify the water later, our goal here is just to get a lowered pH and the murky, viscous water that’s common in these environments.

You can speed up the process with boiling to make a “tea” and then add that to the tank as well. Just make sure to let it cool to room temperature before adding it.

3. Cycle and Test

Hopefully, you know how to cycle a tank, if not the process is pretty simple:

  1. Add a tiny pinch of food to the tank each morning and evening.
  2. Check the ammonia levels in the tank with your kit daily. It will show up after 2-3 days and then fade after another week or so. Wait until ammonia is at 0ppm.
  3. Begin testing for nitrites instead. Nitrites generally pass faster than ammonia but not always. Keep testing until you can no longer detect nitrite.
  4. Begin testing for pH and nitrates daily. Test pH at roughly the same time daily, it will swing a bit with the natural cycle of the tank. pH ranges from 4.5-6.5 are acceptable for a blackwater tank. 4.5 is the beginning of the “extreme” range and most aquarists will be better off at 5.0 to 5.5

When the water chemistry is right, you can begin adding your fish slowly.

4. Maintenance

Maintaining a blackwater tank is only a little bit more complicated than any other tank.

Mainly, you need to do the following:

  • Weekly 25% water changes using RO or DI water. Tap water will mess with your pH balance and should be avoided.
  • Replace leaf litter or cones every 2-3 months.
  • Continue to regularly check pH. Acidic tanks are more prone to swings than alkaline tanks.

And now you’re ready to start keeping the tank and learning more!

Stocking Blackwater Tanks

Blackwater tanks have some pretty extreme conditions. You’ll want to make sure that the fish you want to keep are going to be fine.

You’ll also need to think through plant choices more carefully. That said, let’s take a look at what you can do.

Fish for Blackwater Tanks

The good news is that you have plenty of fish to choose from when you set up a blackwater tank. It’s not as restrictive as most specialized aquariums like brackish tanks.

A huge portion of the fish that we find in pet stores is naturally from blackwater regions.

Tetras are among the most common. Neons, for instance, have trouble breeding in alkaline water and love blackwater environments. It can even make their colors a bit brighter.

Discus can be kept in blackwater. They’re often considered the very high end of freshwater fishkeeping, with the attendant price. These fish do very well in a blackwater tank, which is closer to their natural environment. Since the water needs to be acidic for their health anyways, blackwater is a great option.

Speaking of cichlids, the various species of small Rams also do really well in blackwater tanks. The majority of Dwarf Cichlids prefer a blackwater environment.

For cleanup, Corydoras Catfish and Otocinclus Catfish are both compatible with this environment as well. 

You can also use Plecostomus in these tanks, although they’re not quite as fond of the environment as some of the other options. You may need to watch the tank a bit better with a Pleco, since they tend to tear up wood and constantly stir the substrate.

Finally, you also have Betta and Gourami, both of which prefer blackwater environments. That goes for virtually every species of labyrinth fish within their family.

And, of course, you have the Pirhana family. While the Red Belly Pirhana is restricted in many places, other related fish include Pacu and 

Blackwater environments just aren’t all that toxic. It’s just a low pH environment with a bit of stuff in the water. Still, you should evaluate other fish before you add them.

The essential quality you’re looking for is just the ability to tolerate acidic water.

If your tank is kept in a higher range of pH, in the 6.0-7.0 range, the list includes even more. Livebearers, barbs, and cichlids can all mesh well with the environment.

While fish do very well in these environments, there is a caveat when it comes to stocking your fauna.

Invertebrates for Blackwater Tanks

Forget the usual fare of invertebrates for blackwater tanks. There will be no Ramshorn or Mystery Snails in your tank, even Cherry Shrimp are a no-go once the pH goes under 6.0.

The problem is that the acids in the water attack the shells of freshwater invertebrates. For snails or shrimp, this means that their shell or exoskeleton is breaking down constantly. Since blackwater tanks also have minimal hardness, there’s also no calcium they can intake to heal from the damage.

If you do accidentally place one of these creatures in a blackwater tank the results are going to be a slow death. Snails have the best tell, their shell will begin to thin and lose their coloration, and may last longer than Cherry Shrimp.

Fortunately, there are a few creatures that are specifically adapted to blackwater environments.

Bee Shrimp, or Crystal Red Shrimp, are your best bet for shrimp. These species do well in mildly acidic water but keep it above 5.0 for the best results. Bee Shrimp are harder to keep than Cherry Shrimp, but the biggest problem (lower pH) is already taken care of in a blackwater tank.

For those who really like a challenge, these tanks can also support Bamboo Shrimp. 

Malaysian Trumpet Snails, one of my favorites in planted tanks, seem to be well adapted to the conditions.

Various reports on other snails and shrimp show up. I advise caution if your pH is 6.0 or lower, but some snails of other species will be fine. The main problem is that they’ll never have great shells in a blackwater tank since there’s such a low level of calcium.

Plants for Blackwater Tanks

Plants in blackwater tanks can be rather controversial. It’s a different environment than the one that we keep our plants in normally, and the water chemistry matters a bit more than it does with the fish you’re keeping.

If you’re in the 6.5+ range, which is really more of a tannin-containing tank than true blackwater, you can use whatever you’d like. When you get under that, things change.

I’m not aware of any wide studies done on plants that can be kept in an acidic blackwater environment. This makes it a bit hard to recommend plants, especially since people seem to get different results. It depends largely on the pH of the tank.

I know from experience the following plants will do well in a 5.5-6.0 pH environment:

  • Anubias Species- Do well with low light anyways. Tied to driftwood they seem to thrive in blackwater environments, even growing more quickly than in a neutral tank in my experience.
  • Java Ferns- Does great in blackwater environments, with similar results to Anubias.
  • Java Moss- Grows well, just as invasive and hard to remove as it is in any other tank.
  • Amazon Sword- May need a bit more lighting than normal to maintain fast growth. The rest of the Echinodorus sp. should be fine as well.
  • Cryptocoryne Species- Crypt melt will occur when you place a Cryptocoryne species in a blackwater tank but they grow back with vigor once established.

I’ve seen many other plants growing in these tanks, but I can say with confidence the above will work fine in a truly acidic environment.

There is one thing you have to be aware of when planting in a blackwater tank: you’re going to need to fertilize heavily and supplement with CO2 for long-term success.

Break out every trick you know to pump nutrients into your plants and keep them growing and you’ll be fine. I recommend upgrading your lights as soon as possible.

The bottom line is that not every plant does well in a blacklight tank, but there are more of them than most people think. You’ll have to work harder than normal, but the right plants will work beautifully as an accent to your blackwater aquarium.

Blackwater Chemistry Differences

There are some curious quirks in the chemistry of a low pH tank that not everyone is aware of. The vast majority of aquaria are in a neutral or alkaline environment, so there’s a bit to learn.

Ammonium Production

Under 6.0 pH, for instance, the vast majority of ammonia in the tank is converted to ammonium. Ammonium is actually non-toxic for fish even at extreme levels. This is important, since the health of your bacteria changes with the pH of the tank.

This sounds great, especially for those who insist on doing fish-in cycles. Unfortunately, the reality of a blackwater tank is much more complex than “ammonia doesn’t matter.” Once the pH goes back over 6.0, say during a water change, it converts back into toxic ammonia.

The lesson is that blackwater tanks need to be stable in order for the animals to thrive. The wild environments may have larger swings, but your tank needs to be dialed in until you know that it’s safe.

Blackwater tanks cover such a huge variety of pH readings that it’s sometimes hard to generalize the chemistry. In the wild, some of these environments reach extreme pH levels that change things again. For instance, under 3.0 pH it appears that fungi take over for the suppressed bacteria in the nitrogen cycle.

Activated Carbon

There’s a wide range of opinions on the use of activated carbon in filtration for a blackwater tank. One of the compounds that get bound in activated carbon is the tannins that we’re using to darken our water.

That’s not a ringing endorsement of activated carbon, but it can be overcome.

Upfront: I remove all activated carbon from my systems when running a blackwater tank but it’s up to you to decide what’s right for your situation.

Activated carbon works by being incredibly porous, which allows it to catch a lot of contaminants that end up in our water. It’s an essential piece of the filtration puzzle in many tanks, as long as it’s maintained.

And that’s part of the problem. With all of the tannin and particulate matter in a blackwater tank, you’ll need to replace your carbon with regularity. Activated carbon which has reached its absorption limit does function as a limited biological filter.

This can cause further problems since replacing the activated carbon removes those bacteria from the system, creating an imbalance that has to be corrected. While being corrected, there’s going to be a sticking point with either ammonia or nitrite during the breakdown of waste matter.

It’s just a bit unstable, and one of the key components of successfully keeping a blackwater tank (especially a low-volume tank) is creating a stable environment.

On the other hand, activated carbon is great for stripping chlorine, ammonia, and some other chemicals from the tank that are commonly introduced. The main advantage is usually water clarity, but it’s a bit counterproductive in this case for obvious reasons.

If your filter allows it, you may want to cycle in activated carbon for a week every month. This consists of just adding the media and removing it at the end of the week, not allowing it to become a thriving colony of bacteria.

One big reason to use activated carbon is its affinity for phenols. Phenol compounds are what produce the foul smell that develops in some aquaria and a regular cycle can keep the smell down.

It’s your choice, and it will only make a minor difference either way. Just be aware that activated carbon will make it harder to achieve a blackwater effect.

Carbon Dioxide Setups in Blackwater Aquariums

Carbon dioxide is great for plants, but it presents a few extra challenges in a blackwater tank.

The main one is that it changes the pH of your water. Blackwater tanks are all about stability, at least within reason, and you’ll have to accommodate the pH swing that CO₂ creates in your tank.

That’s easy enough to adjust to. Simply take a few measurements throughout the day to figure out how the CO₂ is causing the pH to swing. This will usually cause a less acidic environment at night.

The other problem is that it’s easy to overdose CO₂ in a true blackwater tank. This will just increase the pH without actually doing anything for your plants, and it can even hurt your fish if the levels are too high.

CO₂’s role in the tank is mediated by light. There’s just less photosynthesis going on in a blackwater tank when compared to a normal tank that has the same size and lighting. Blackwater blocks light, reducing the CO₂ needed.

If your fish are gasping for air at the surface or acting lethargic then you should immediately dial back the CO2. No harm done if you catch it in time, but a serious imbalance of carbon dioxide and oxygen can kill fish over time.

The solution is simple: stay on top of the pH readings in your tank and use less CO₂ than you would in a regular planted tank.

A Clearer Picture of Dark Water

There’s a lot of mystique around blackwater aquariums, I hope that I’ve managed to dispel some of it. These aquariums have a wide range of different qualities but you’ll find that they stay healthy as long as you’re on top of your water chemistry. The huge choice of fish fit for this environment is just a bonus.

So, are you ready to dive headfirst into a blackwater tank?