Black ghost knifefish and Doctor fish
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
- 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 14 cm, an adult Doctor fish 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
pH ranges overlap only across 7–7.5, which is tighter than most tap-water supplies hold naturally.
Derived from: environment.ph overlap width
Black ghost knifefish defends a territory and both animals use the same part of the tank, so give the territory-holder a defensible area away from the main swimming space.
Derived from: traits.territoriality and zone 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
Both species occupy the bottom of the tank, so allow for the shared space when judging the footprint.
Derived from: zone and traits.activity on both records
Black ghost knifefish needs soft sand it can sift, while Doctor fish wants rock structure. Both are achievable in one tank, but the substrate choice is not negotiable for the sand-sifter — sharp gravel damages barbels and the damage is permanent.
Derived from: traits.substrateNeed on both records
Why it can work
- Hardness requirements overlap at 8–12 °dGH.
- Doctor fish needs a group of at least 5. 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.
- 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 5 Doctor fish, 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
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 | 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 | One animal at the edge of its range | The shared band 7–7.5 pH sits at the top of one animal's range, so that animal is permanently at its limit even though the overlap looks generous. |
| Hardness | One animal at the edge of its range | The shared band 8–12 °dGH sits at the bottom of one animal's range, so that animal is permanently at its limit even though the overlap looks generous. |
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 | One animal at the edge of its range. The shared band 7–7.5 pH sits at the top of one animal's range, so that animal is permanently at its limit even though the overlap looks generous. |
| Hardness | Derived from stored traits | One animal at the edge of its range. The shared band 8–12 °dGH sits at the bottom of one animal's range, so that animal is permanently at its limit even though the overlap looks generous. |
| Body size | Derived from stored traits | Black ghost knifefish reaches about 50 cm against 14 cm for Doctor fish — a ratio of 3.6. 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 | Doctor fish needs at least 5. 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 | Both occupy the bottom of the tank, so the shared space is under pressure. |
| 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 | 24–26 °C | Aim for the middle of the shared band. |
| pH | 7–7.5 | Stability matters more than the exact figure. |
| Hardness | 8–12 °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 Doctor fish.
- 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
Doctor fish
Hard alkaline water
Tigris-Euphrates spring-fed and geothermal pools
300–900 µS/cm · moderate flow · moderate oxygen
- TemperatureBlack ghost knifefish 24–28°CDoctor fish 18–26°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 6–7.5Doctor fish 7–8
Both inside their range from 7–7.5 — 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.
- General hardnessBlack ghost knifefish 2–12 dGHDoctor fish 8–20 dGH
Both inside their range from 8–12 dGH — 40% 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.
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.