Siamese fighting fish and Harlequin rasbora
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
- 85 L
- Minimum length
- 90 cm
- Factors considered
- 7
Concerns
Siamese fighting fish 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 24–27 °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
Siamese fighting fish 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
Why it can work
- pH requirements overlap at 6–7.5.
- Hardness requirements overlap at 2–12 °dGH.
- Harlequin rasbora 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.
Conditions
Most interesting pairings are conditional on something. These are the things the keeper can actually change.
- At least 8 Harlequin rasbora, counted as one group
- At least 85 L with a footprint of 90 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
One of the better-supported betta community pairings, and the outcome still depends on the individual betta rather than on anything about the rasboras. Plan a tank that works if the betta turns out to be intolerant, because a proportion of them are.
What would change it: A larger footprint with broken sightlines lowers the risk without removing it. A betta that has already shown sustained pursuit will not be talked out of it.
Factor by factor
- Fin shape and nipping riskReported
Harlequin rasboras are reported as among the least fin-oriented of the common shoalers, and the difference from a tiger barb or a serpae tetra is consistent enough across accounts to plan around. It is a lower risk rather than no risk.
- WaterDerived
Both are Sundaland soft-water fish and the overlap is genuine rather than a compromise, which is unusual in a betta pairing and worth more than it looks.
- Betta temperamentReported
Reports range from total indifference to sustained pursuit, and the variation is between individual bettas rather than between tanks. No species-level verdict resolves this.
| Depends on | How much |
|---|---|
| Tank size and dimensions | significant |
| Life stage | none |
| The individual animal | decisive |
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 24–27 °C is usable, though it does not sit centrally for both animals. |
| 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 2–12 °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 24–27 °C is usable, though it does not sit centrally for both animals. |
| 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 2–12 °dGH sits centrally in both ranges, with room to drift. |
| Body size | Derived from stored traits | Adult sizes are comparable at about 7 cm and 4.5 cm, so neither is a mouthful for the other. |
| Predation | Derived from stored traits | Highest predation risk in either direction is "low". |
| 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 | Evidence-backed | No nipper-and-target combination between these two. Reported: Harlequin rasboras are reported as among the least fin-oriented of the common shoalers, and the difference from a tiger barb or a serpae tetra is consistent enough across accounts to plan around. It is a lower risk rather than no risk. |
| 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 | Harlequin rasbora 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 "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 60 L and 60 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: none. See the documented record. |
| Individual variation | Evidence-backed | Individual variation: decisive. |
Environmental overlap
| Parameter | Shared range | Reading |
|---|---|---|
| Temperature | 24–27 °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 Siamese fighting fish or Harlequin rasbora.
- 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.
Siamese fighting fish
Rice paddy, ditch and drain
central Thai floodplain and paddy
80–500 µS/cm · still flow · low oxygen
Harlequin rasbora
Peat swamp forest
10–70 µS/cm · still flow · low oxygen
- TemperatureSiamese fighting fish 24–28°CHarlequin rasbora 22–27°C
Both inside their range from 24–27°C — 75% 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.
- pHSiamese fighting fish 6–7.5Harlequin rasbora 5–7.5
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 hardnessSiamese fighting fish 2–15 dGHHarlequin rasbora 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.