Black ghost knifefish and Neon tetra
Can these two share a tank? The assessment below is derived from stored behavioural and environmental traits, not from a compatibility chart.
Should not be attempted. At least one factor is expected to cause harm.
- Minimum volume
- 675 L
- Minimum length
- 210 cm
- Factors considered
- 8
Concerns
Black ghost knifefish reaches about 50 cm and takes fish small enough to swallow. At roughly 3.5 cm, an adult Neon tetra is inside that range. This is usually discovered as fish quietly disappearing overnight rather than as visible aggression.
Derived from: traits.predation.smallFish, adultSize on both records, and the stored body form and gape of both
The shared band 24–26 °C sits at the top of one animal's range, so that animal is permanently at its limit even though the overlap looks generous. The overlap sits at the extreme of at least one animal's range, so that animal is permanently at its limit. A non-empty overlap is not the same as a suitable one.
Derived from: environment.temperature range position on both records
Black ghost knifefish becomes markedly more aggressive while spawning. A pairing that has been peaceful for months can change within a day, so plan for where the other animal goes if it does.
Derived from: traits.breedingAggression
Why it can work
- pH requirements overlap at 6–7.5.
- Hardness requirements overlap at 2–10 °dGH.
- Neon tetra needs a group of at least 8. Kept in smaller numbers it is not simply lonely — shoaling species held below their threshold show chronic stress behaviour, colour loss and, in several species, increased aggression towards tankmates.
- The two occupy different levels of the tank, which reduces direct competition for space.
- Black ghost knifefish is active mainly after dark, so the two overlap less than the stocking list suggests. Feed the nocturnal animal after lights-out or it will be outcompeted.
Conditions
Most interesting pairings are conditional on something. These are the things the keeper can actually change.
- At least 8 Neon tetra, counted as one group
Derived, or documented?
These are different claims and are shown separately. The verdict above is derived from stored traits — arithmetic on AquariumHQ's own records. Whether the pairing is actually reported to work is a separate question, and for most pairs the honest answer is that we hold nothing.
Environmental compatibility
Derived from stored traits
Temperature, pH, hardness, size and behavioural traits compared between the two records. Sound as far as it goes, and it is not evidence that the pairing works.
Behavioural evidence
Widely reported in practice
Reported consistently across aquarium practice without a single citable source. Treated as real but not as measurement.
AquariumHQ's overall position
A poor match on a delay, and one that is unusually hard to diagnose because the losses happen in the dark. A tank that seems to be losing a tetra a fortnight to no obvious cause is usually a tank with a nocturnal predator in it.
What would change it: Nothing about the tank. Tankmates for a black ghost knifefish have to be chosen on adult size, and the threshold is larger than most community fish reach.
Factor by factor
- Nocturnal predation on sleeping fishReported
The knifefish hunts at night by electrolocation, and a sleeping tetra does not need to be caught so much as found. Losses are attributed to almost anything else because nobody sees them happen.
- Adult gapeDerived
A mature black ghost reaches fifty centimetres. A neon tetra is inside its gape long before that.
- Water and temperamentDerived
Soft warm water suits both, and the knifefish is not aggressive in any sense that matters. This is entirely a feeding relationship.
| Depends on | How much |
|---|---|
| Tank size and dimensions | minor |
| Life stage | decisive |
| The individual animal | minor |
How good is the overlap?
Whether two ranges intersect is a necessary condition, not evidence. What matters is WHERE the shared band sits: an overlap at the top of one animal's range and the bottom of the other's is a forced compromise however wide it looks, and it is the case a simple intersection test cannot see.
| Dimension | Assessment | What that means here |
|---|---|---|
| Temperature | One animal at the edge of its range | The shared band 24–26 °C sits at the top of one animal's range, so that animal is permanently at its limit even though the overlap looks generous. |
| pH | Workable | The shared band 6–7.5 pH is usable, though it does not sit centrally for both animals. |
| Hardness | Comfortable for both | The shared band 2–10 °dGH sits centrally in both ranges, with room to drift. |
Factor by factor
Every factor states what it rests on. Most are derived from the traits stored on both records — sound arithmetic on AquariumHQ's own data, and not a report about how the pairing actually goes. Where a factor says 'not known', the data is missing and saying so matters, because a missing factor and a favourable one look identical once both are absent.
| Factor | Basis | Finding |
|---|---|---|
| Temperature | Derived from stored traits | One animal at the edge of its range. The shared band 24–26 °C sits at the top of one animal's range, so that animal is permanently at its limit even though the overlap looks generous. |
| pH | Derived from stored traits | Workable. The shared band 6–7.5 pH is usable, though it does not sit centrally for both animals. |
| Hardness | Derived from stored traits | Comfortable for both. The shared band 2–10 °dGH sits centrally in both ranges, with room to drift. |
| Body size | Derived from stored traits | Black ghost knifefish reaches about 50 cm against 3.5 cm for Neon tetra — a ratio of 14.3. Size disparity drives opportunistic predation independently of temperament. |
| Predation | Evidence-backed | Highest predation risk in either direction is "near-certain". Reported: The knifefish hunts at night by electrolocation, and a sleeping tetra does not need to be caught so much as found. Losses are attributed to almost anything else because nobody sees them happen. |
| Territoriality | Derived from stored traits | Strongest territoriality across the pair is "moderate". |
| Breeding aggression | Derived from stored traits | Breeding aggression reaches "moderate". A pairing peaceful for months can change within a day of a spawn. |
| Fin-nipping | Derived from stored traits | No nipper-and-target combination between these two. |
| Aggression | Derived from stored traits | Strongest intraspecific aggression across the pair is "strong". It bears on the pairing because a fish under pressure from its own kind redirects. |
| Social requirements | Derived from stored traits | Neon tetra needs at least 8. A shoaling species held below its threshold shows chronic stress and, in several species, increased aggression towards tankmates. |
| Group-size dependency | Derived from stored traits | The outcome depends on group size as much as on the pairing: under-grouped shoalers nip more and hide more. |
| Tank zone | Derived from stored traits | The two use different levels of the tank, which reduces direct competition for space. |
| Activity level | Derived from stored traits | Activity levels are compatible. |
| Food competition | Derived from stored traits | Different feeding strategies, so direct competition is limited. |
| Risk to shrimp and snails | Derived from stored traits | Highest risk either animal poses to dwarf shrimp or snails is "near-certain". |
| Effect on plants | Derived from stored traits | Neither animal damages planting. |
| Tank dimensions | Derived from stored traits | The more demanding of the two needs at least 500 L and 180 cm of length before anything else is in the tank. Volume alone does not describe a home for a fish that swims in straight lines. |
| Life stage | Evidence-backed | Life-stage dependency: decisive. A mature black ghost reaches fifty centimetres. A neon tetra is inside its gape long before that. |
| Individual variation | Evidence-backed | Individual variation: minor. |
Environmental overlap
| Parameter | Shared range | Reading |
|---|---|---|
| Temperature | 24–26 °C | Aim for the middle of the shared band. |
| pH | 6–7.5 | Stability matters more than the exact figure. |
| Hardness | 2–10 °dGH | Check your source water before committing. |
What this assessment cannot tell you
- Individual temperament varies, particularly in cichlids and in bettas, and can override a favourable verdict for Black ghost knifefish or Neon tetra.
- The assessment is for the pair in isolation. A third species can change the outcome in either direction.
- Both animals are assessed at adult size, not at the size they are sold.
- A verdict is a statement about probability, not a guarantee.
Where the two water bands meet
Derived from the ranges AquariumHQ publishes for each species. The compatibility engine reads the same numbers; this is what they look like drawn on one axis.
Temperature (°C)
Scale: 0 to 36 °C.
pH (pH)
Scale: 4 to 9.5 pH.
Hardness (°dGH)
Scale: 0 to 30 °dGH.
Why the bands sit where they do
The overlap above is arithmetic. This is where the two numbers came from — the kind of water each animal is recorded from, and how much room that leaves.
These two animals come from different kinds of water, and that is where the arithmetic below comes from rather than from anything about their temperaments.
Black ghost knifefish
Clearwater lowland river
20–100 µS/cm · moderate flow · near saturation oxygen
Neon tetra
Blackwater
shaded upper-Amazon tributaries, black and clear water mixed
8–40 µS/cm · low flow · moderate oxygen
- TemperatureBlack ghost knifefish 24–28°CNeon tetra 20–26°C
Both inside their range from 24–26°C — 50% of the narrower animal’s range.
A real window, and a narrow one. The tank has to be held inside it, and there is little room for the drift that happens between water changes.
- pHBlack ghost knifefish 6–7.5Neon tetra 4–7.5
Both inside their range from 6–7.5 — 100% of the narrower animal’s range.
A wide window. Both animals are inside their own range across most of it, and the tank can be run without either sitting at an edge.
- General hardnessBlack ghost knifefish 2–12 dGHNeon tetra 1–10 dGH
Both inside their range from 2–10 dGH — 89% of the narrower animal’s range.
A wide window. Both animals are inside their own range across most of it, and the tank can be run without either sitting at an edge.
Computed from the ranges AquariumHQ publishes for each species, each of which records what it rests on — a source value, a habitat inference, a physiological requirement or a trade convention. Most are habitat inferences, and the species pages say which.