Guppy and Molly
Can these two share a tank? The assessment below is derived from stored behavioural and environmental traits, not from a compatibility chart.
Requirements align and no behavioural conflict is expected.
- Minimum volume
- 150 L
- Minimum length
- 100 cm
- Factors considered
- 6
Why it can work
- Temperature requirements overlap comfortably at 24–28 °C, with room for both animals to sit inside their preferred range.
- pH requirements overlap at 7.5–8.5.
- Hardness requirements overlap at 15–25 °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.
- Molly needs a group of at least 4. 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 4 Molly, 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
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
Standard hard-water community stock and usually fine in a tank with room. The molly is the larger, more robust animal and the guppy is the one that finds out; the pairing gets worse as the tank gets smaller and better as the water gets harder.
What would change it: Length. Mollies are active fish and a 60 cm tank does not give a harassed guppy anywhere to be.
Factor by factor
- WaterDerived
Both want hard alkaline water and the molly wants it harder — 15–30 dGH against the guppy's 8–25. The overlap is real and the molly is at the bottom of its range in water that suits the guppy.
- Size and harassmentReported
An adult molly is two to three times a guppy's length and is reported to harass and occasionally nip them. The reports are consistent for sailfin mollies in particular.
- Fry predation, both waysDerived
Both species eat their own and each other's fry. In a mixed livebearer tank the recruitment problem solves itself, which most keepers regard as a feature.
| Depends on | How much |
|---|---|
| Tank size and dimensions | significant |
| Life stage | minor |
| The individual animal | significant |
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 24–28 °C sits centrally in both ranges, with room to drift. |
| pH | Comfortable for both | The shared band 7.5–8.5 pH sits centrally in both ranges, with room to drift. |
| Hardness | Workable | The shared band 15–25 °dGH is usable, though it does not sit centrally for both animals. |
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 24–28 °C sits centrally in both ranges, with room to drift. |
| pH | Derived from stored traits | Comfortable for both. The shared band 7.5–8.5 pH sits centrally in both ranges, with room to drift. |
| Hardness | Derived from stored traits | Workable. The shared band 15–25 °dGH is usable, though it does not sit centrally for both animals. |
| Body size | Derived from stored traits | Molly reaches about 15 cm against 6 cm for Guppy — a ratio of 2.5. Size disparity drives opportunistic predation independently of temperament. |
| Predation | Derived from stored traits | Highest predation risk in either direction is "none". |
| Territoriality | Derived from stored traits | Neither animal holds a territory. |
| 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 | Derived from stored traits | Strongest intraspecific aggression across the pair is "none". 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; Molly needs at least 4. 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 150 L and 100 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: significant. |
Environmental overlap
| Parameter | Shared range | Reading |
|---|---|---|
| Temperature | 24–28 °C | Aim for the middle of the shared band. |
| pH | 7.5–8.5 | Stability matters more than the exact figure. |
| Hardness | 15–25 °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 Guppy or Molly.
- 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.
Guppy
No wild referent
variable flow · not characteristic oxygen
Molly
Hard alkaline water
300–900 µS/cm · moderate flow · moderate oxygen
- TemperatureGuppy 22–28°CMolly 24–28°C
Both inside their range from 24–28°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.
- pHGuppy 7–8.5Molly 7.5–8.5
Both inside their range from 7.5–8.5 — 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 hardnessGuppy 8–25 dGHMolly 15–30 dGH
Both inside their range from 15–25 dGH — 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.
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.