Poor match4 concerns

Siamese fighting fish and Guppy

Can these two share a tank? The assessment below is derived from stored behavioural and environmental traits, not from a compatibility chart.

VerdictPoor matchhigh confidence

Workable only in unusual circumstances. Not a pairing to plan around.

Minimum volume: 85 L
Minimum length: 90 cm
Factors considered: 8

Concerns

moderateph

pH ranges overlap only across 7–7.5, which is tighter than most tap-water supplies hold naturally.

Derived from: environment.ph overlap width

moderateaggression

Siamese fighting fish harasses slow-moving or long-finned tankmates, and Guppy 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

moderatebreeding aggression

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

minorterritory

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

Temperature requirements overlap comfortably at 24–28 °C, with room for both animals to sit inside their preferred range.
Hardness requirements overlap at 8–15 °dGH.
Guppy 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.
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 6 Guppy, 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

Environmental compatibility is moderate — a compromise rather than a match. Behavioural compatibility depends on the individual betta and on the sex of the guppies. AquariumHQ records this as conditional, and notes that the hardness compromise is a real cost even when the behaviour works.

Factor by factor

Male guppy colouration and finnageReported
Male guppies carry long, brightly coloured fins, which is the specific stimulus reported to provoke bettas. Female guppies are reported to be a considerably lower risk, which makes this a pairing whose outcome depends on which guppies rather than whether guppies.
Guppies nipping the bettaReported
The risk runs both ways. Guppies are reported to nip trailing fins, and the betta is a target.
HardnessDerived
Guppies are hard-water fish and bettas are not. The overlap is workable and neither animal is in its preferred water at the compromise.

What the answer depends on. A single cell in a compatibility chart cannot hold any of these.
Depends on	How much
Tank size and dimensions	significant
Life stage	minor
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.

Each dimension assessed for the quality of the overlap, not merely its existence.
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	Forced compromise — neither animal is where it wants to be	The shared band 7–7.5 pH sits at the top of one animal's range and the bottom of the other's. Both would be kept at their limits to accommodate the other.
Hardness	Workable	The shared band 8–15 °dGH is usable, though it does not sit centrally for both animals.

Forced compromise — neither animal is where it wants to be

Both animals would be kept at the edge of their ranges to accommodate the other. Nothing here is a recommendation; it is the arithmetic of two incompatible requirements meeting at a point.

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.

3 evidence-backed16 derived0 not known

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	Forced compromise — neither animal is where it wants to be. The shared band 7–7.5 pH sits at the top of one animal's range and the bottom of the other's. Both would be kept at their limits to accommodate the other.
Hardness	Derived from stored traits	Workable. The shared band 8–15 °dGH is usable, though it does not sit centrally for both animals.
Body size	Derived from stored traits	Adult sizes are comparable at about 7 cm and 6 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: Male guppies carry long, brightly coloured fins, which is the specific stimulus reported to provoke bettas. Female guppies are reported to be a considerably lower risk, which makes this a pairing whose outcome depends on which guppies rather than whether guppies.
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	Guppy 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	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: minor. See the documented record.
Individual variation	Evidence-backed	Individual variation: decisive.

Environmental overlap

Parameter	Shared range	Reading
Temperature	24–28 °C	Aim for the middle of the shared band.
pH	7–7.5	Stability matters more than the exact figure.
Hardness	8–15 °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 Siamese fighting fish or Guppy.
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.

The two species

Siamese fighting fishBetta splendensThe fish most often kept in conditions that would be unacceptable for anything else. The labyrinth organ lets a betta breathe air, which is why it survives in unfiltered bowls — surviving low oxygen is not the same as needing little space, and a betta in a heated, filtered, planted tank behaves like a different animal.GuppyPoecilia reticulataDomestic guppies are hard-water fish, which is the opposite of what most people assume. They are also among the most widely introduced fish on Earth — released deliberately for mosquito control and now established across the tropics with documented harm to native species.