Breeding Report: The Black Emperor Tetra (Nematobrycon amphiloxus)

Breeding Report: The Black Emperor Tetra (Nematobrycon amphiloxus)

The Black Emperor Tetra (Nematobrycon amphiloxus) is a fish with a story as dark and rich as its velvety flanks. This breeding report details my experiences in spawning and raising this strikingly beautiful characin, based on a lineage that may well trace back to the earliest exports by William “Fred” Kyburz from Colombia’s Pacific slopes.

Origins of a Regal Black Beauty

The Black Emperor Tetra (Nematobrycon amphiloxus) is more than just a darkened variant of the common Emperor Tetra. It carries with it a murky, intriguing backstory. One that mirrors the development of the South American ornamental fish trade itself.

If you’re going to breed a fish like this, it’s worth understanding where it came from. That context adds weight to the project, even if it unfolds in a 12-litre tank tucked into a garden shed.

A Swiss Plant Collector in Colombia

The story begins in the late 1950s with William “Fred” Kyburz, a Swiss-born journalist-turned-plant-collector. Specializing in philodendrons and anthuriums, Kyburz settled in the misty highlands near Bitaco, Colombia. It was here, in the catchment of the Rio Calima, a southern tributary of the Rio San Juan, that he collected the fish that would later captivate generations of aquarists.

Though not a fish collector by trade, Kyburz sent shipments abroad, including both Nematobrycon palmeri and what we now call amphiloxus. At the time, both forms were likely lumped together.

Mutation or Mixture?

In the 1960s, Kyburz began exporting fish from Cali, Colombia. One of his customers was Rosario La Corte, a legendary aquarist based in New Jersey. La Corte noted that soon after importing Emperor Tetras, some offspring began appearing with notably darker flanks. He initially assumed these were mutants; a spontaneous genetic variation within the same species.

“The black colour of these fish varied greatly in intensity,” observed La Corte.

This caught the attention of Dr. Stanley Weitzman, who, upon visiting La Corte’s fishroom in 1970, identified three distinct forms of Nematobrycon. Together with Dr. Fink, he published a 1971 paper describing Nematobrycon amphiloxus as a new species. But even then, he noted that preserved specimens lost their striking black colours and closely resembled the standard palmeri type.

Two Rivers, One Fish?

What may have caused this diversity? The answer might lie in history.

In colonial times, a canal was dug linking the headwaters of the Rio Atrato and Rio San Juan. This artificial waterway likely allowed two once-isolated populations of Emperor Tetras to intermix, producing hybrids that Kyburz later collected.

“Colour has and always will be a poor feature on which to positively identify a fish,” wrote Weitzman and Fink.

The black form we know today may be a geographical colour variant, a subspecies, or even a domestic strain fixed over decades of selective breeding.

A Legacy of Line Breeding

What we do know is that La Corte selectively bred the darkest individuals. Over time, the Black Emperor Tetra became established in the hobby, especially in the United States. His breeding efforts, and those of others, helped preserve this unique lineage for over 55 years.

Even today, commercial breeders likely continue to select for stronger black coverage. That’s likely why all of my own offspring turned out jet black, with no throwbacks to the metallic violet hues of standard Emperors.

More Than Just a Colour Variant

So is Nematobrycon amphiloxus a true species? Or a colour morph of N. palmeri? Taxonomists remain unsure. But to me, it’s more than a taxonomic puzzle, it’s a living legacy. A fish shaped not only by mountain streams and jungle ponds, but also by the quiet selectivity of careful breeders.

Watching my young, peach-bellied females deepen in colour over time, and seeing my males grow more refined and elegant, reminded me that beauty in fishkeeping often lies in the details, and in the stories they carry with them.

Broodfish Selection & Conditioning

Choosing and preparing the right broodstock is the cornerstone of any successful tetra breeding project, and this is especially true for the stunning Black Emperor Tetra, Nematobrycon amphiloxus. My journey began with a small group of juveniles purchased from a local fish shop. They were still immature at the time, likely no more than three to four months old, but showed promising signs of health and vibrant coloration.

Sexing the Black Emperors

Sexing this species is relatively straightforward compared to many other tetras, which makes them a great choice for those new to characin breeding:

• Males are larger, more robust, and display a striking jet-black coloration across the flanks. Their eyes are vividly blue, and the most distinctive trait is the elongated central caudal fin ray, which begins developing at just a few months of age. With maturity, this central ray grows well beyond the top and bottom lobes of the tail, forming a true “swordtail” appearance.
• Females, while smaller and slightly more reserved in temperament, are just as striking. They also exhibit black coloration, although they retain a slender peachy-yellow or tan ventral stripe that runs along the belly. Their eyes are yellow, a helpful and consistent indicator of sex even at a glance.

Interestingly, older females in my breeding group also began developing a modest central tail extension, although never as prominent as that seen in males.

Conditioning the Broodfish

After purchase, the six young Black Emperors (three males, three females) were quarantined in a sparsely decorated 25-litre tank for three weeks. This quarantine not only ensured their health, but also allowed the fish to acclimatize slowly and begin filling out on a high-quality diet.

Once out of quarantine, they were introduced into a heavily planted 100-litre community aquarium—my “odds and sods” tank—shared with a mix of tetras from other breeding projects. No attempt was made to separate sexes during this period. The goal was simply to allow the group to grow, mature, and establish a natural rhythm before any breeding attempts were made.

Over several months, the Black Emperors flourished. Their coloration deepened further—females becoming darker and more imposing in their own right—and the males became increasingly bold and expressive. Inter-male displays became a common sight, particularly during feeding or when holding small territories in the community setup.

Diet and Environmental Cues

As with most tetra species, diet played a critical role in conditioning the fish for breeding. Their daily rations included:

• Frozen copepods
• Small frozen bloodworms
• Frozen adult brine shrimp
• Occasional live bloodworms
• Leftover baby brine shrimp from other fry tanks

This variety, especially the live or recently hatched offerings, triggered not only physical conditioning but also behavioural cues suggestive of readiness to spawn. There’s something unmistakably invigorating about the movement of live food in the water—particularly for tetras—and I firmly believe that this acts as a synchronising stimulus for spawning.

Water Parameters During Conditioning

• Community Tank TDS: ~70 ppm
• pH: Slightly acidic to neutral
• Temperature: Ambient conditions with stability (no artificial lighting used)
• Light Cycle: Natural light from skylights in my office fish room, which I believe plays a significant role in establishing normal circadian and spawning rhythms. Artificial lights, even programmable LEDs, never seem to match the nuanced signals of natural dawn and dusk.

Despite being raised in hard water (my tap supply sits on a chalk aquifer), the Emperors showed no hesitation in maturing or displaying pre-spawning behaviour. Regular 25% water changes and a consistent food regimen proved far more valuable than obsessing over exact mineral content or water chemistry at this stage.

Spawning Tank Setup

When it comes to spawning delicate tetras like Nematobrycon amphiloxus, the setup of the breeding tank can make the difference between a failed attempt and a hundred healthy fry. For this project, I used a 12-litre aquarium, set up in my outdoor office where it could remain undisturbed and benefit from natural light cycles through skylights. Something I’ve come to believe plays a significant role in triggering successful spawning behaviour.

Preparing the Tank

The tank was first thoroughly cleaned with tap water and a sponge, followed by a rinse and soak using white vinegar to remove any potential residues or mineral deposits. After a final rinse and a day to dry, the tank was placed on a foam mat for insulation and support.

I then lined the outside of the tank’s walls and back with repurposed brown cardboard. This simple trick reduces light and visual stress while helping the fish feel more secure in the smaller space. Once filled, the tank was situated in a quiet, low-traffic corner of the room.

Water Parameters

• Volume: ~8 litres initially (to keep depth <15 cm)
• Water Type: Primarily Reverse Osmosis (RO) with a small amount of treated tap water to add trace minerals
• Temperature: 25°C (warmed in a saucepan before being added to the tank)
• pH: ~6.2
• TDS: 20–30 ppm
• GH/KH: Undetectable using standard liquid and dip test strips

Using mostly RO water creates a soft, clean, and low-TDS environment, conditions often associated with natural tetra spawning habitats. The shallow water also improves the fry’s chance of successfully inflating their swim bladders post-hatch.

Substrate & Leaf Litter

An Indian Almond Leaf (Catappa) was prepared by:

1. Rinsing under the tap
2. Blanching in boiling water for a few seconds
3. Folding and tucking it behind the heater

The leaf serves several purposes:

• Mimics natural habitat with tinted water
• Slowly releases tannins and humic substances with antifungal properties
• Acts as a grazing surface for fry and infusorians
• Suppresses light, creating a more secure environment for both adults and eggs

Mesh Grid for Egg Protection

A false bottom was fashioned using a piece of oven chip tray mesh, the kind sold for helping crisp up frozen French fries. This semi-rigid plastic mesh was cut to fit snugly above the base silicone bead of the aquarium.

Its purpose:

• Prevent egg predation by the adults
• Allow eggs to fall safely through the grid and remain undisturbed
• Enable egg inspection with a small flashlight via gaps between mesh and glass

Getting the mesh to fit just right is essential, especially with smaller tetra species. You don’t want to find one of the broodfish beneath it after all your careful preparation.

Spawning Medium & Filtration

For spawning medium, I used a dense clump of Java Moss sourced from another tank. It was rinsed briefly under the tap to remove any snails or unwanted microfauna and then teased out to fill approximately two-thirds of the tank’s volume. The moss provides:

• Spawning substrate
• Cover from aggression
• Shelter for fry once hatched

Filtration was provided by a small circular sponge filter connected to an air pump and run at a gentle “tick-over” level. This provided sufficient biological filtration without creating strong water movement that might dislodge eggs or exhaust newly hatched larvae.

A preset submersible heater (15 watts, set at 25°C) completed the setup.

Timing the Transfer

The spawning tank was filled and left to settle for about five hours before the fish were transferred, around 8 p.m., just after dusk. I transferred five fish (two males and three females), although I was only able to net two of the males from the well-planted community tank.

The broodfish were not acclimated to the new tank conditions, as the temperature and pH were close enough, and I believe the abrupt shift may help trigger spawning, much like the sudden onset of rainy season conditions in the wild.

Spawning Behaviour & Egg Care

Spawning activity in Nematobrycon amphiloxus, the Black Emperor Tetra, can be both subtle and surprising. Unlike some more overtly boisterous tetras, the spawning of Amphiloxus often takes place in the quiet, early hours, when no one is watching.

Natural Light and an Early Start

Much of the spawning behaviour I observed, or rather inferred, took place before I’d even stepped foot in the room. My outdoor office, where the spawning tank was located, benefits from natural skylight exposure, and I’m convinced this played a critical role. While many aquarists rely solely on artificial lighting, I’ve had repeated success allowing natural dawn light to initiate spawning. On the morning following the transfer of the broodstock, the skylight was already closed by the time I entered around 6 a.m., suggesting most of the activity occurred during first light, probably around 4 a.m. in high summer.

This quiet start may mirror the fish’s natural behaviour in the wild, spawning at first light in shaded, tannin-stained backwaters of Colombia’s upper Rio San Juan and Rio Calima.

Behaviour in the Spawning Tank

Although I did not witness the spawning directly, I observed a notable behavioural levelling once the five fish were introduced into the 12-litre tank. The two males and three females had been living communally, where males regularly flared and postured, especially towards one another. But inside the tighter confines of the spawning tank, these territorial instincts appeared to dissolve under the weight of hormonal triggers.

Within 24 hours, the fish had completed spawning. One of the females, who had appeared distinctly gravid when viewed from above in the community tank, was noticeably slimmer. The status of the other two females was harder to determine, but it’s possible they participated to a lesser extent.

The Eggs

Around 100 eggs were visible beneath the mesh grid, lying safely on the bare glass and among the tangled strands of Java moss. Fertility was remarkably high, and unlike with some tetras, I did not notice a significant number of cloudy or unfertilized eggs.

The eggs were small, slightly adhesive, and scattered rather than clustered. As with most tetras, there was no parental care, which is why the mesh grid is so essential. Even with the best-conditioned broodstock, they will eat their own eggs if given the chance.

Post-Spawning Procedure

Once spawning ceased, I immediately removed the adults using a two-net method. A fine mesh net to gently herd them into a larger waiting net. They were then re-acclimatised back to the community tank over a 30–40 minute period, using the float-and-drip method with tank water slowly added to a jug holding the fish.

The Java moss was shaken vigorously in tank water and rinsed several times in a separate container to release any remaining eggs that might be hidden or attached. This step is crucial for maximising yield.

Finally, I removed the mesh grid and covered the tank with a towel to block out strong light,just in case the eggs are light-sensitive. Although Amphiloxus eggs don’t appear to be as delicate in this respect as, say, Hyphessobrycon species, I prefer to err on the side of caution.

Early Water Management

To counteract the release of proteins and organics into the water during spawning, I performed a gentle water change immediately after removing the broodstock. Using a 4mm air line and cane skewer, I siphoned off surface water only, carefully avoiding the egg-laden bottom. Roughly half the tank volume (about 4 litres) was removed and replaced with temperature-matched RO water.

From this point on, the water level was intentionally kept shallow (<15 cm) to help the fry reach the surface and inflate their swim bladders once hatched. The sponge filter was suspended just under the surface using a clothes peg on the air line, minimising current while maintaining surface agitation to prevent slicks from forming.

Feeding Fry & Early Rearing

Raising Nematobrycon amphiloxus fry, is a deeply rewarding experience, especially when the process runs smoothly. In this section, I’ll walk through the methods I used to ensure a high survival rate, successful swim bladder inflation, and a strong start in life for the fry.

Hatch Timing and Swim Bladder Development

Roughly 3.5 to 4.5 days after the eggs were laid, the tiny larvae hatched. At this stage, they remained relatively immobile. Clinging to the tank walls, folded Catappa leaf, heater, or sponge filter. Their bodies were translucent, their eyes large, and they had prominent yolk sacs.

One of the critical phases during this time is swim bladder inflation. To support this, I maintained the tank at a shallow water depth of no more than 15 centimetres. This allows the larvae to easily reach the surface and gulp atmospheric air, helping them to inflate their developing swim bladders. Failure at this stage leads to “belly sliders”; larvae unable to lift themselves off the bottom, doomed to fail. Sadly, these must be culled.

The air-driven sponge filter was hoisted close to the surface using a clothes peg on the airline tubing. This simple trick helped ensure gentle surface movement without creating too much circulation, allowing the fry to reach the surface with minimal effort.

First Feeding: Timing is Everything

The timing of the first feeding is absolutely crucial. I’ve found that fry typically become ready to feed shortly after they begin exhibiting more coordinated, sustained swimming, usually 4 days post-spawn.

There are two broad strategies I’ve employed for early feeding:

1. Liquifry No.1 (Interpet)

When I used Liquifry instead of live paramecia, I dosed the tank before the fry became free-swimming, just as the instructions suggest. Liquifry doesn’t feed the fry directly; instead, it promotes the growth of infusoria and biofilm, forming a natural grazing field in the tank.

I mix the drops in a small test tube filled with water from the tank, give it a good shake, then distribute it evenly over the surface. This avoids dumping concentrated doses directly over the fry.

2. Live Paramecia, Golden Pearls, and Microworms

When I have live paramecium available, I add a few drops as soon as I see fry sticking to vertical surfaces or hiding under the heater or almond leaf. In the earliest hours, I may also include:

• Golden Pearl powder (5–50 microns): A small “pinhead” amount mixed with tank water and shaken vigorously before dosing.
• Microworms or banana worms: Added sparingly for subtle movement to stimulate feeding responses.

Transition to Baby Brine Shrimp

A day or two after the initial feeds begin, I start dribbling in San Francisco Bay strain baby brine shrimp (BBS). This is my preferred food for young tetra fry. Nothing beats the nutrition of freshly hatched nauplii.

Using a pipette, I add just a small amount and monitor the fry under a flashlight. Seeing orange bellies is the moment of confirmation that feeding has been successful. Once this happens, you’re through the most delicate phase.

At this point, I increase feedings to 2-3 times daily, alternating between BBS, fine powdered foods, and an occasional cloud of suspended microworms if I’m away for work or short on time.

Stirring Up the Biofilm

When raising fry on infusoria and powdered feeds, I found it useful to manually stir up the detritus and biofilm forming on the tank’s glass and bottom. I’d simply rub a finger across the glass or raise and lower the sponge filter to create some gentle circulation. This re-suspends food particles and microfauna, making them more visible and accessible to the fry.

Tetra fry often snap reflexively at drifting movement, and this technique helps elicit that vital first feeding behaviour.

Cleaning and Water Management

In the early stages, I remove 1 litre of water per day using airline tubing or a turkey baster and replace it with 1.5 to 2 litres of temperature-matched RO water. This gently increases the tank volume while keeping the water quality high.

After about a month, once the fry begin to look like miniature adults, I gradually transition to a 50:50 RO to tap water mix, slowly raising the TDS from the original 20–30 ppm to around 60 ppm. This helps harden the fry for life beyond the breeding setup and prepares them for future transfers.

The Catappa leaf is left in place throughout early rearing, acting as a biofilm substrate, shelter, and natural tonic. Once the fry are confidently eating BBS, the tank becomes easier to clean, and the biofilm is less critical.

Growth & Transfer Stages

Successfully hatching Nematobrycon amphiloxus is just the beginning. Raising those tiny fry into healthy juveniles requires careful observation, feeding discipline, and timely transfers to progressively larger tanks. In this chapter, I’ll walk you through the grow-out stages as I experienced them, along with lessons learned along the way.

The First Month: From Larva to Miniature Emperor

For the first four weeks, the fry remained in their original 12-litre breeding tank. As described earlier, this tank started with just 8 litres of soft, low-TDS RO water. Over time, through small, incremental water changes, I gradually increased both the volume and mineral content.

Daily cleaning involved removing about 1 litre of water using airline tubing or a turkey baster. This was replaced with 1.5 to 2 litres of temperature-matched pure RO water, carefully dripped into the tank. This slow approach allowed the tank volume to rise while keeping water parameters stable.

By the end of the first month, the fry were visibly pigmented, with developing caudal fin extensions evident in some individuals. At this stage, the fry looked like miniature versions of the adult fish—slender-bodied, with deep black flanks beginning to show.

Feeding Routine and Growth Acceleration

Once all the fry were taking newly hatched baby brine shrimp (BBS) readily, growth accelerated. I maintained a varied diet that included:

• BBS: Fed 2–3 times daily.
• Frozen copepods: Excellent nutrition and variety.
• Golden Pearls (5–50 microns): Used occasionally between BBS feeds.
• Microworms and banana worms: Reliable standby foods, especially when I was away or short on time.
• Crushed flake and granulated fry food: Introduced around the one-month mark for dietary variety.

I found that regular feedings and keeping the fry well-fed, but not overfed, were key to uniform growth. After feeding, the air-driven sponge filter was briefly lowered to re-circulate suspended particles, then returned to a higher position to reduce current and help conserve fry energy.

Transfer to Larger Tanks

Around week five, I moved the fry from the 12-litre tank to a 40-litre grow-out tank. This marked a significant transition in their development. The transfer process was simple: using a jug, I gradually mixed water from the new tank with water from the fry tank over 30 to 40 minutes to avoid osmotic shock.

After another four to five weeks, the juveniles—now nearly the size of the original parents when I purchased them, were moved again, this time to a 64-litre tank. By this stage, their body shape, eye colour, and caudal fin extensions were becoming much more distinct, allowing for the early identification of sex.

Tip: Always move fry to larger quarters before growth begins to slow. Undersized tanks limit potential and encourage waste build-up, which even a sponge filter can’t keep up with long-term.

Gradual Hardening of Water

As the fry matured, I slowly increased the hardness of their water by adding a higher percentage of treated tap water to each water change. The transition looked something like this:

• Weeks 1–4: 100% RO water
• Weeks 5–8: ~25% tap, 75% RO
• After Week 8: 50:50 mix of tap and RO water

This shift increased the TDS from 20–30 ppm in the early stages to around 60–80 ppm. The pH also rose slightly, moving from 6.2 toward neutral. These gradual changes prepared the fish for life in standard aquarium conditions without shocking their osmoregulatory systems.

Age	Tank Size	Diet Highlights	TDS	Notes
Day 1–7	12L	Liquifry, infusoria	20 ppm	Shallow water for swim bladder inflation
Day 7–30	12L	BBS, Golden Pearls, microworms	25–40 ppm	Pigmentation appears, early sex traits
Week 5–8	40L	BBS, flake, frozen copepods	50–60 ppm	Rapid growth, transferred after acclimation
Week 9–12	64L	Varied diet, more crushed flake	60–80 ppm	Juveniles reach parent size

Reflections & Advice for Breeders

Breeding Nematobrycon amphiloxus, the Black Emperor Tetra, has been one of the more rewarding and surprisingly manageable breeding projects I’ve undertaken in recent years. From the initial courtship to watching the jet-black juveniles mature into regal adults, the experience reaffirmed that even a relatively modest setup, paired with a patient and thoughtful approach, can yield excellent results.

A Species that Rewards Subtlety and Routine

One of the key takeaways from this breeding experience is just how responsive these fish are to gentle environmental cues rather than aggressive intervention. Spawning occurred without the need for drastic temperature swings, dosing peat, or chasing elusive pH values. Instead, a modest drop in hardness and TDS, coupled with ample live and frozen food and reduced ambient light, proved sufficient to trigger a successful spawning event.

Keeping things simple and stable, while nudging the fish in the right direction, allowed their natural instincts to do the rest.

“With commercially raised characins like these, I’ve learned not to get too hung up on testing. In the time it takes to run a full suite of tests, I could have done a 50% water change.”

That sentiment sums up much of my philosophy, clean water, consistent feeding, and observing behaviour often yield more than any gadget ever will.

Advice to First-Time Tetra Breeders

If you’re considering Amphiloxus Black as your first tetra breeding project, here are a few key bits of advice:

• Don’t obsess over pH: Focus instead on TDS and general water softness. RO water blended with tap water gives you the best of both worlds.
• Use natural cues: Low light, leaf litter, and a steady feeding schedule seem to do more than synthetic triggers.
• Feed live food regularly: Baby brine shrimp (BBS) is irreplaceable once fry are large enough. Microworms and golden pearls make excellent stop-gaps.
• Keep your setup modest and replicable: A simple 12L tank, false bottom, sponge filter, and almond leaf proved completely adequate.
• Don’t wait too long to upsize: Fry grow best when transferred to roomier tanks before they start competing for space and oxygen.

On Raising Fry Without Paramecium

Many breeders assume success is only possible with cultured infusoria, and while that helps, it’s not mandatory. In this case, I raised the entire batch without dedicated paramecium cultures. Interpet Liquifry No.1, seasoned sponge filters, and the natural microfauna in the tank carried the fry through their most fragile stage. With carefully dosed microfoods and patience, you can achieve similar results.

The Joy of Line-Breeding and Sharing Success

With a 90–95% survival rate from the first successful spawn, I was able to distribute dozens of juvenile fish to fellow hobbyists and friends. Watching these fish thrive in other tanks is as rewarding as raising them myself. Their distinctive black colouration, iridescent blue or yellow eyes, and elegant fin extensions make them stand out among even the most vibrant tetra species.

I’ve kept the original broodfish, now well over two years old, and they remain vigorous and beautiful, a testament to the robustness of this strain.

Final Thoughts

There’s something deeply satisfying about seeing a batch of fry you raised by hand mature into strong, healthy fish. Nematobrycon amphiloxus may not be the flashiest name in the hobby, but their understated beauty, peaceful temperament, and ease of breeding make them an ideal choice for aquarists looking to dip their toes into characin reproduction.

“They’re not just a good beginner’s fish. They’re a good reminder that even simple setups and well-worn techniques can still deliver magic.”

If you’re a breeder looking for a rewarding, manageable, and beautiful project, give the Black Emperor Tetra a go. You may find yourself, like me, enchanted by these regal black jewels of the Rio Calima.

— Mike Lee

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