Scissortail rasbora and Severum
Can these two share a tank? The assessment below is derived from stored behavioural and environmental traits, not from a compatibility chart.
Can work, but only if the stated conditions are met.
- Minimum volume
- 450 L
- Minimum length
- 165 cm
- Factors considered
- 7
Concerns
Severum 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
Temperature requirements overlap at 24–26 °C, though the band does not sit centrally for both animals. Aim for the middle of it and keep the heater accurate.
Derived from: environment.temperature on both records
Severum 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
Both species occupy the midwater 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
- pH requirements overlap at 6–7.
- Hardness requirements overlap at 2–12 °dGH.
- Scissortail rasbora needs a group of at least 6. 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.
Conditions
Most interesting pairings are conditional on something. These are the things the keeper can actually change.
- At least 6 Scissortail rasbora, counted as one group
- At least 450 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
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 | Workable | The shared band 24–26 °C is usable, though it does not sit centrally for both animals. |
| pH | Comfortable for both | The shared band 6–7 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 | Workable. The shared band 24–26 °C is usable, though it does not sit centrally for both animals. |
| pH | Derived from stored traits | Comfortable for both. The shared band 6–7 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 | Severum reaches about 25 cm against 12 cm for Scissortail rasbora — a ratio of 2.1. Size disparity drives opportunistic predation independently of temperament. |
| Predation | Derived from stored traits | Highest predation risk in either direction is "high". |
| Territoriality | Derived from stored traits | Strongest territoriality across the pair is "moderate". |
| Breeding aggression | Derived from stored traits | Breeding aggression reaches "strong". 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 "moderate". It bears on the pairing because a fish under pressure from its own kind redirects. |
| Social requirements | Derived from stored traits | Scissortail rasbora needs at least 6. 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 midwater 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 "near-certain". |
| Effect on plants | Derived from stored traits | Severum eats plants, which constrains the aquascape rather than the pairing. |
| Tank dimensions | Derived from stored traits | The more demanding of the two needs at least 300 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 | 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 | 6–7 | 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 Scissortail rasbora or Severum.
- 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.
Both animals come from clearwater lowland river, which is why the bands below sit where they do.
Scissortail rasbora
Clearwater lowland river
20–100 µS/cm · moderate flow · near saturation oxygen
Severum
Clearwater lowland river
20–100 µS/cm · moderate flow · near saturation oxygen
- TemperatureScissortail rasbora 22–26°CSeverum 24–29°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.
- pHScissortail rasbora 6–7.5Severum 5.5–7
Both inside their range from 6–7 — 67% 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 hardnessScissortail rasbora 2–15 dGHSeverum 1–12 dGH
Both inside their range from 2–12 dGH — 91% 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.