Black ghost knifefish and Moonlight gourami
Can these two share a tank? The assessment below is derived from stored behavioural and environmental traits, not from a compatibility chart.
Workable only in unusual circumstances. Not a pairing to plan around.
- Minimum volume
- 750 L
- Minimum length
- 225 cm
- Factors considered
- 10
Concerns
Black ghost knifefish reaches about 50 cm and takes fish small enough to swallow. At roughly 18 cm, an adult Moonlight gourami is inside that range. This is usually discovered as fish quietly disappearing overnight rather than as visible aggression. Moonlight gourami is deep-bodied and Black ghost knifefish has a moderate, so Moonlight gourami stops being a possible mouthful SOONER than the usual rule suggests — at around 27% of Black ghost knifefish's length rather than 40%. The warning stands on what this predator is recorded as doing rather than on the geometry, because a fish that is awkward to swallow whole can still be attacked.
Derived from: traits.predation.smallFish, adultSize on both records, and the stored body form and gape of both
Black ghost knifefish harasses slow-moving or long-finned tankmates, and Moonlight gourami fits that description. Persistent harassment shows up as hiding, clamped fins and refusal to feed long before it shows up as injury.
Derived from: traits.predation.harassment and traits on the second record
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
Moonlight gourami 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
Flow preferences differ (moderate against still). Aim for the middle and provide a sheltered corner.
Derived from: traits.flowPreference on both records
Why it can work
- Temperature requirements overlap comfortably at 24–28 °C, with room for both animals to sit inside their preferred range.
- pH requirements overlap at 6–7.5.
- Hardness requirements overlap at 2–12 °dGH.
- 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 750 L with a footprint of 225 cm or more, before counting anything else in the tank
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
No behavioural evidence held
AquariumHQ holds no behavioural evidence for this pairing. The verdict is derived entirely from stored traits — it is arithmetic on our own records, not a report about how the pairing actually goes.
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 | Comfortable for both | The shared band 24–28 °C sits centrally in both ranges, with room to drift. |
| pH | Comfortable for both | The shared band 6–7.5 pH sits centrally in both ranges, with room to drift. |
| Hardness | Comfortable for both | The shared band 2–12 °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 | Comfortable for both. The shared band 24–28 °C sits centrally in both ranges, with room to drift. |
| pH | Derived from stored traits | Comfortable for both. The shared band 6–7.5 pH sits centrally in both ranges, with room to drift. |
| Hardness | Derived from stored traits | Comfortable for both. The shared band 2–12 °dGH sits centrally in both ranges, with room to drift. |
| Body size | Derived from stored traits | Black ghost knifefish reaches about 50 cm against 18 cm for Moonlight gourami — a ratio of 2.8. Size disparity drives opportunistic predation independently of temperament. |
| Predation | Derived from stored traits | Highest predation risk in either direction is "near-certain". |
| 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 | Neither animal requires a group. |
| Group-size dependency | Derived from stored traits | The outcome does not depend on group size for either animal. |
| 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 | Not known | AquariumHQ holds no record of whether this pairing changes as either animal matures. Stored traits describe adults, so a pairing that fails at adult size looks identical to one that never fails. |
| Individual variation | Not known | AquariumHQ holds no record of how much this pairing varies between individuals. For several species — bettas and cichlids especially — that variation decides the outcome and a species-level verdict cannot express it. |
Environmental overlap
| Parameter | Shared range | Reading |
|---|---|---|
| Temperature | 24–28 °C | Aim for the middle of the shared band. |
| pH | 6–7.5 | Stability matters more than the exact figure. |
| Hardness | 2–12 °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 Moonlight gourami.
- 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
Moonlight gourami
Lowland floodplain
40–300 µS/cm · still flow · low oxygen
- TemperatureBlack ghost knifefish 24–28°CMoonlight gourami 24–30°C
Both inside their range from 24–28°C — 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.
- pHBlack ghost knifefish 6–7.5Moonlight gourami 6–7.5
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 dGHMoonlight gourami 2–18 dGH
Both inside their range from 2–12 dGH — 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.
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.