Rainbow shark and Red-tailed black shark
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
- 270 L
- Minimum length
- 165 cm
- Factors considered
- 5
Concerns
Both animals hold strong territories. Two strongly territorial residents in one tank means continuous boundary pressure unless the footprint is large enough to give each an independent area with sightline breaks.
Derived from: traits.territoriality on both records
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
Why it can work
- Temperature requirements overlap comfortably at 22–27 °C, with room for both animals to sit inside their preferred range.
- pH requirements overlap at 6.5–7.8.
- Hardness requirements overlap at 5–15 °dGH.
Conditions
Most interesting pairings are conditional on something. These are the things the keeper can actually change.
- A long tank with hardscape dividing it into visually separate territories
- At least 270 L with a footprint of 165 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
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, and one that works in a shop tank and then does not. Both species are sold as juveniles that tolerate each other, and the intolerance arrives with maturity rather than with any change in the tank.
What would change it: Nothing available in a domestic aquarium. This is a pairing to plan against rather than to manage, and the practical answer is one shark of either species per tank.
Factor by factor
- Congeneric aggressionReported
Two Epalzeorhynchos in one tank is the pairing this genus is least able to tolerate. Each treats the other as the closest possible rival, and the conflict does not settle into a boundary the way disputes with unrelated fish sometimes do.
- Identical territory shapeDerived
Both defend a floor patrol route. Two identical shapes cannot be separated by furnishing a tank differently.
| Depends on | How much |
|---|---|
| Tank size and dimensions | significant |
| 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 | Comfortable for both | The shared band 22–27 °C sits centrally in both ranges, with room to drift. |
| pH | Comfortable for both | The shared band 6.5–7.8 pH sits centrally in both ranges, with room to drift. |
| Hardness | Comfortable for both | The shared band 5–15 °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 22–27 °C sits centrally in both ranges, with room to drift. |
| pH | Derived from stored traits | Comfortable for both. The shared band 6.5–7.8 pH sits centrally in both ranges, with room to drift. |
| Hardness | Derived from stored traits | Comfortable for both. The shared band 5–15 °dGH sits centrally in both ranges, with room to drift. |
| Body size | Derived from stored traits | Adult sizes are comparable at about 15 cm and 15 cm, so neither is a mouthful for the other. |
| Predation | Derived from stored traits | Highest predation risk in either direction is "none". |
| Territoriality | Derived from stored traits | Strongest territoriality across the pair is "strong". |
| Breeding aggression | Derived from stored traits | Neither becomes notably more aggressive while spawning. |
| Fin-nipping | Derived from stored traits | No nipper-and-target combination between these two. |
| Aggression | Evidence-backed | Strongest intraspecific aggression across the pair is "strong". It bears on the pairing because a fish under pressure from its own kind redirects. Reported: Two Epalzeorhynchos in one tank is the pairing this genus is least able to tolerate. Each treats the other as the closest possible rival, and the conflict does not settle into a boundary the way disputes with unrelated fish sometimes do. |
| 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 | 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 | Both are omnivores feeding in the same part of the tank. |
| Risk to shrimp and snails | Derived from stored traits | Highest risk either animal poses to dwarf shrimp or snails is "high". |
| 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 180 L and 120 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. See the documented record. |
| Individual variation | Evidence-backed | Individual variation: minor. |
Environmental overlap
| Parameter | Shared range | Reading |
|---|---|---|
| Temperature | 22–27 °C | Aim for the middle of the shared band. |
| pH | 6.5–7.8 | Stability matters more than the exact figure. |
| Hardness | 5–15 °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 Rainbow shark or Red-tailed black shark.
- 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.
Rainbow shark
Lowland floodplain
40–300 µS/cm · still flow · low oxygen
Red-tailed black shark
Hill stream and rapids
50–300 µS/cm · high flow · near saturation oxygen
- TemperatureRainbow shark 22–27°CRed-tailed black shark 22–27°C
Both inside their range from 22–27°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.
- pHRainbow shark 6.5–7.8Red-tailed black shark 6.5–7.8
Both inside their range from 6.5–7.8 — 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 hardnessRainbow shark 5–15 dGHRed-tailed black shark 5–15 dGH
Both inside their range from 5–15 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.