Understanding Apteronotus Sexual Maturity
Breeding Apteronotus is a manageable endeavor, though it requires attention to specific details. These fascinating fish reach sexual maturity at approximately 18 months of age. In aquarium conditions, adult specimens typically grow to 35-40 centimeters, while wild individuals can reach up to 50 cm. For housing and breeding a pair of Apteronotus, you’ll need an aquarium of at least 150-200 liters.
Sexual Dimorphism in Apteronotus
Sexual dimorphism in these fish is quite subtle. Females are generally slightly smaller than males and display a more rounded belly. Males may develop a fatty hump on the nuchal region. In my experience, determining the sex of juvenile fish is practically impossible—you’ll need to wait until they fully mature.
Interestingly, researchers have discovered differences in the frequency of electrical signals generated by males and females: females produce signals at 1000-1150 Hz, while males generate signals at 850-1000 Hz. However, for practical breeding purposes, this fact remains largely theoretical.
The first successful captive breeding of Apteronotus was achieved by Leningrad (St. Petersburg) aquarists in 1990, who also shed light on the reproductive physiology of this species.
Behavior and Maintenance Requirements
Apteronotus are nocturnal fish. During daylight hours, they prefer resting in secluded spots, often in unusual positions—from lying on their side to upside down. While omnivorous, they thrive best on live foods such as bloodworms, tubifex, and earthworms. A quality diet rich in live foods promotes reproductive readiness in breeding stock. Though some have reportedly fed them dried daphnia and boiled beef, I recommend against experimenting with such diets when preparing fish for spawning.
Compatibility Considerations
Toward other species, Apteronotus show little aggression. They generally don’t bother fry either, at least when adequately fed. However, they typically display aggression toward conspecifics, fighting over food or hiding spots for daytime rest. These skirmishes rarely result in fatalities, but injuries do occur. The most common damage during such encounters is a bitten-off caudal fin, which fortunately regenerates quickly, demonstrating their remarkable regenerative capacity.
Setting Up the Breeding Tank
For spawning, prepare a separate aquarium of 150-200 liters. Apteronotus favor water flow and deposit eggs in areas with the strongest current. In aquariums, these are typically locations near bottom filters and pumps.
Spawned eggs can get sucked into filters, making retrieval difficult. I recommend partitioning off a section of the aquarium with coarse mesh or perforated plexiglass. Holes or mesh openings should be at least 3 millimeters. Position the pump to create constant flow through the partition toward the separated corner. This way, eggs will be drawn into the safe zone but won’t reach the pump itself, settling on the bottom instead.
Water Parameters for Breeding
| Parameter | Value |
|---|---|
| Temperature | 25-26°C |
| Hardness | 6° dGH |
| pH | 6.0-6.5 |
| Salt addition | 1 tbsp per 20-30 L |
| Aeration | Gentle, continuous |
Salting the water is advisable—use one tablespoon of salt per 20-30 liters. Both marine and table salt work effectively. Understanding water hardness is crucial for maintaining optimal breeding conditions.
Spawning Triggers
In nature, Apteronotus breed during the rainy season, so mimicking these conditions helps. Gradually lower the water level over one month, simulating drought. Then, for 2-3 weeks, add fresh soft water in streams for 3-4 hours daily, simultaneously reducing hardness and raising water level. This method effectively triggers spawning.
For breeding, select one female and two males—this composition is considered optimal. If you have a confirmed male-female pair, they will eventually spawn. The male and female, usually antagonistic toward each other, will declare a truce during spawning. Positioning themselves in a suitable location, they’ll release eggs in small batches. Spawning continues for several hours, during which the female may deposit up to 500 eggs.
Caring for Apteronotus Eggs
The eggs are large—2-2.5 millimeters—light gray, transparent, sometimes with a yellowish tint. Remove any whitened eggs promptly. After spawning concludes, immediately remove the parents, as they may consume their own eggs.
Transfer eggs to an incubation container with water depth of 15-20 centimeters, diffused lighting, and gentle aeration. Incubation averages 48 hours, though it may accelerate to 32-36 hours or extend to 96. The exact factors remain unclear—possibly minor temperature fluctuations or water chemistry variations.
Raising Apteronotus Fry
Larvae are large with prominent yolk sacs and yellow coloration. As the yolk sac absorbs, juveniles acquire black pigmentation. Remember their elevated oxygen requirements—regularly change about one-third of the water volume and maintain gentle aeration.
Fry actively swim at night but hide during daylight. They may wedge themselves so tightly into narrow crevices that they cannot escape and perish. Therefore, I hesitate to recommend specific substrates for Apteronotus breeding tanks. Plain sand might work. Alternatively, simply shade the aquarium during daytime—young fish seek crevices due to innate photophobia. Eliminating potentially fatal hiding spots while removing the light stimulus that drives this behavior reduces losses.
Feeding Schedule
Once larvae transform into fry, begin feeding. The best starter foods are cyclops and artemia nauplii. Subsequently, transition to small crustaceans. Upon reaching 1.5 centimeters length, introduce small bloodworms.
Juvenile Aggression
Be aware that Apteronotus display conspecific aggression from the fry stage onward. While eliminating overly tight crevices, still provide adequate spacious shelters. As fry grow, separate them into larger accommodations.
Planning Your Breeding Project
Considering adult size, plan ahead for their future. Specifically, identify potential buyers or recipients for these fish. Remember that a pair of adult Apteronotus requires an aquarium of at least 200 liters—providing adequate space for normal existence exceeds most aquarists’ individual capacity.
Keeping growing juveniles in undersized aquariums will stunt their growth, preventing them from achieving their impressive natural dimensions. Additionally, given their quarrelsome nature, they’ll likely fight constantly over territory, caves, or food. Experience breeding other species, such as Betta fish, demonstrates that planning is key to success.
As you can see, breeding Apteronotus is quite achievable with appropriate equipment and patience. I hope this guide has addressed some of your questions and helps you decide whether breeding Black Ghost Knifefish suits your home aquarium goals.
Article updated: 2026-08-31
