Elephant-nose Knifefish (Sternarchorhynchus oxyrhynchus)

What is an Elephant-nose Knifefish?

The Elephant-nose Knifefish (Sternarchorhynchus oxyrhynchus) is one of the most remarkable and unusual freshwater fish available to specialist aquarists. A member of the ghost knifefish family Apteronotidae, it propels itself entirely by undulating a long anal fin that runs the length of its belly — the same fluid, ribbon-like movement seen in the better-known Black Ghost Knifefish. What sets S. oxyrhynchus apart from every other knifefish in the trade is its extraordinary tubular snout, which can equal or exceed the length of the rest of the head. This structure is packed with electroreceptors and is used to probe leaf litter, fine sediment and crevices for hidden invertebrate prey.

Like all apteronotids, this species generates a weak electric field produced by a specialised electric organ running along its flanks. The fish uses this field for orientation in zero-visibility blackwater, for detecting prey, and for communicating with others of its kind. Adults reach up to 35 cm (14 in) in total length and are exclusively nocturnal: during daylight hours they remain motionless inside driftwood caves or dense tangles of roots, emerging only after lights out to forage across the bottom. This is genuinely a fish for the committed specialist — rated Hard for good reason.

Where does the Elephant-nose Knifefish come from?

Wild specimens are collected across the Amazon Basin — principally from Brazil, Peru and Colombia — where the species inhabits slow-moving rivers, flooded forest margins, blackwater tributaries and heavily shaded forest streams. The water in these environments is characteristically very soft (1–8 dGH), warm (24–29 °C / 75–84 °F) and acidic (pH 5.5–7.0), heavily stained with tannins leached from submerged wood and decaying vegetation, and extremely low in dissolved minerals. Substrates are typically fine dark sand overlaid with deep accumulations of leaf litter.

This fish does not adapt to harder or more alkaline water the way many other South Americans do. Attempting to keep it in moderately hard, neutral tap water tends to produce chronic low-grade stress that shortens its life significantly. S. oxyrhynchus appears in the trade only intermittently — it is not mass-bred commercially and most available fish are wild-caught — so sourcing healthy specimens from a specialist importer is essential.

What size tank does an Elephant-nose Knifefish need?

A single adult requires a minimum of 200 L (53 gal) of water volume, and a footprint of at least 150 × 50 cm gives a 35 cm fish adequate territory to patrol at night without constantly colliding with glass. Larger is better: a 300–400 L (80–106 gal) setup provides more stable water chemistry and makes it easier to maintain the low-nutrient, blackwater conditions the species needs.

The aquascape should be dim, dense and cave-rich. Use substantial pieces of driftwood with hollow interiors or create overhanging root tangles to give the fish secure daytime refuges — without convincing hiding places this fish will refuse to eat and will waste away. A thick layer of dried Indian almond leaves or catappa bark on the substrate releases tannins and humic acids that help replicate Amazonian blackwater. Floating plants such as Amazon frogbit help diffuse overhead light to a level the fish tolerates. Flow from the filter should be gentle: powerful currents are not found in the still forest streams this species calls home, and strong flow is stressful rather than beneficial.

What water parameters does the Elephant-nose Knifefish need?

• Temperature: 24–29 °C (75–84 °F) — stable heat is essential; avoid fluctuations greater than 1–2 °C.
• pH: 5.5–7.0; aim for 6.0–6.8 in practice, the soft acidic end of the range.
• Hardness: 1–8 dGH — very soft water is the goal; reverse-osmosis water blended with tap is the most reliable way to hit this target.
• Ammonia / Nitrite: 0 ppm at all times; the fish is intolerant of poor water quality.
• Nitrate: Keep below 20 ppm with weekly 25–30 % water changes.

The most critical parameter is stability rather than any single number. Wild blackwater habitats change very slowly, and this fish has little physiological tolerance for rapid swings. Condition all replacement water to within 0.5 °C and the same pH before adding it to the tank. Use a quality pH and TDS meter and test regularly — this is not a fish to maintain on guesswork.

What does the Elephant-nose Knifefish eat?

The Elephant-nose Knifefish is a strict carnivore that feeds by probing soft substrates with its elongated snout, detecting and extracting small invertebrates by their faint bioelectric signatures. In captivity it takes small live and frozen foods readily: bloodworms, blackworms, tubifex, white worms, brine shrimp and daphnia are all accepted. Earthworms cut into small pieces can be offered as variety. It will almost never voluntarily take dry pellets or flake food, and attempting to switch it to dry foods as a staple is a reliable path to starvation.

Feed after lights out — this species will not come out to eat while the room is bright. A target feeding of two to three times per week is appropriate for adults; juveniles benefit from more frequent smaller meals. Because the fish is entirely nocturnal and shy, it is easy to underestimate how much it is consuming. Monitor body condition regularly: a healthy Elephant-nose Knifefish should show a gently rounded belly profile, not a sunken, pinched abdomen.

Is the Elephant-nose Knifefish peaceful — and what fish can live with it?

This species is genuinely peaceful toward other fish and poses no threat to tankmates. The compatibility challenges run in the opposite direction: it is the one that needs protection. The Elephant-nose Knifefish is easily intimidated by boisterous or fast-moving species and will be outcompeted for food by any fish that reaches the bottom first. It also cannot be housed with other weakly electric fish — overlapping electric fields from other apteronotids or mormyrids cause chronic interference that disrupts the fish’s ability to sense its environment, leading to severe stress.

Suitable companions are calm, similarly sized, soft-water South American species that occupy other areas of the tank: large tetras (such as altum tetras or bleeding-heart tetras), larger peaceful dwarf cichlids, South American leaf fish kept with caution, or well-chosen catfish that are not aggressive bottom-competitors. Avoid any cichlid that defends territory aggressively, any fast or nippy species, and all other electric fish.

For a curated, filterable list of tested pairings, visit Elephant-nose Knifefish tank mates.

How do you tell a male from a female Elephant-nose Knifefish?

There are no reliable external differences between males and females of S. oxyrhynchus in captivity. Males and females appear identical in body shape, coloration and snout length at all life stages. This is consistent with the frontmatter data and with what is documented in the scientific literature. Determining sex would require internal examination or imaging — neither practical nor worthwhile for the aquarist. Breeding groups are therefore assembled by purchasing multiple individuals and allowing natural pairing, though even this approach has produced very few documented captive spawnings.

Can the Elephant-nose Knifefish be bred in captivity?

Breeding is rated Very Hard and has been achieved only rarely outside of research settings. This is consistent with almost all wild-caught, highly specialist knifefish: the precise water chemistry cues, space requirements, and environmental triggers needed to initiate spawning are poorly understood, and the species does not breed commercially.

For the aquarist willing to attempt it, the general approach for apteronotids involves providing a very large, heavily decorated tank with pristine soft-acid water, a period of seasonal simulation (slight temperature drop and increased rainfall simulation through water change volume), and — critically — confirmed male-female pairs, which cannot be identified visually. Egg scattering behaviour has been described in related species but no detailed spawning account for S. oxyrhynchus in the hobby literature exists at the time of writing. Treat any breeding attempt as an exploratory project rather than an expected outcome.

What diseases affect the Elephant-nose Knifefish?

The Elephant-nose Knifefish is sensitive to several conditions, most of which are preventable through water quality management:

• Ich (white spot / Ichthyophthirius multifiliis): The fish is susceptible, and because it lacks scales on much of its body its tolerance for standard treatments is lower than most species. Prevention through strict quarantine of new fish and stable water temperature is far preferable to treatment.
• Bacterial infections: Open sores or fin damage can become infected quickly in a fish that spends much of its time near the substrate. Maintain pristine water quality and remove any sharp decor that could cause abrasion.
• Starvation and wasting: By far the most common cause of loss in captivity. A fish that is not eating — often because it is stressed by lighting, current or incompatible tankmates — will decline gradually and invisibly. Monitor body condition monthly.
• Osmotic stress: Keeping the fish in water outside its preferred hardness and pH range causes chronic physiological stress that suppresses the immune system and shortens lifespan even when no obvious disease is present.

Health note: medication dosing and disease diagnosis are beyond the scope of a care profile. Scaleless and scale-reduced fish such as knifefish are often sensitive to standard aquarium medications at normal doses. If disease is suspected, confirm symptoms against a veterinary or specialist fish-health source before treating, and use any medication at reduced concentration with close observation.

How long does the Elephant-nose Knifefish live?

With consistent, expert-level care the Elephant-nose Knifefish can live 8–12 years in captivity — a substantial commitment that reflects both its specialist demands and the reward of getting them right. Wild-caught fish occasionally arrive in the trade already mature, so the full potential lifespan from point of purchase may be somewhat shorter. The fish that reach the upper end of this range are invariably those kept in large, well-maintained blackwater biotopes with steady water chemistry, appropriate hiding structure, and regular live or frozen food. Cut corners on any one of these and the fish’s resilience — which is limited — shows quickly.