Why mollies and tetras are usually a water problem
Both are sold as community fish. That phrase describes temperament, and the difficulty here is not temperament — it is that the water one is recorded from is water the other is not. It also is not one answer: some tetras are far closer to the molly than others.
The question
Is 'community fish' a water-chemistry category, and can a molly share a tank with tetras?
The evidence, live
Rendered from AquariumHQ's evidence layer rather than restated in prose, so this page cannot drift from the data behind it. If a value is corrected, what you see here changes.
Why we say this
What each source reports for Molly — general hardness.
What each source reports
| Source | Value | What kind of value | Published at | Checked |
|---|---|---|---|---|
| FishBase | 11–30 °dGH | Tolerated range | Published for this species | 2026-08-11 |
How AquariumHQ read them
Only one source is recorded, so there is nothing to corroborate it against.
One source is recorded, giving 11–30 °dGH. Nothing corroborates it, so the band is reported as that source's position rather than as a consensus.
Last reviewed 2026-08-11
Why we say this
Serpae tetra
What each source reports
| Source | Value | What kind of value | Published at | Checked |
|---|---|---|---|---|
| FishBase | 10–25 °dGH | Tolerated range | Published for this species | 2026-08-11 |
How AquariumHQ read them
Only one source is recorded, so there is nothing to corroborate it against.
One source is recorded, giving 10–25 °dGH. Nothing corroborates it, so the band is reported as that source's position rather than as a consensus.
Why we say this
Harlequin rasbora, a soft-water contrast
What each source reports
| Source | Value | What kind of value | Published at | Checked |
|---|---|---|---|---|
| FishBase | 5–12 °dGH | Tolerated range | Published for this species | 2026-08-11 |
How AquariumHQ read them
Only one source is recorded, so there is nothing to corroborate it against.
One source is recorded, giving 5–12 °dGH. Nothing corroborates it, so the band is reported as that source's position rather than as a consensus.
Why we say this
Molly pH
What each source reports
| Source | Value | What kind of value | Published at | Checked |
|---|---|---|---|---|
| FishBase | 7.5–8.2 pH | Tolerated range | Published for this species | 2026-08-11 |
How AquariumHQ read them
Only one source is recorded, so there is nothing to corroborate it against.
One source is recorded, giving 7.5–8.2 pH. Nothing corroborates it, so the band is reported as that source's position rather than as a consensus.
“Community fish” is a statement about behaviour
It means a fish unlikely to eat or harass its tankmates. It says nothing whatever about pH, hardness or temperature, and it is used in retail as though it did — as though anything in the community section can go in the community tank.
Two fish can be perfectly peaceful and still be unable to share water. That is the case here, and no amount of temperament assessment will surface it.
What the records say
FishBase records the molly from water at pH 7.5–8.2 and 11–30 dGH. That is hard, alkaline water, and it is consistent with the brackish and coastal habitats the species occupies.
The soft-water species are recorded from the opposite end, and the harlequin rasbora's figures are rendered live below rather than quoted. FishBase publishes no hardness value for either the cardinal or the neon tetra, so the verdicts against those two are computed from AquariumHQ's own advised ranges instead — which is why they are described here rather than shown.
Run the overlap arithmetic and the verdicts are not close. Against the cardinal tetra, both pH and hardness come out impossible — no shared band exists at all. Against the neon tetra and the harlequin rasbora, hardness is impossible and pH is a forced compromise, meaning any value chosen sits at one animal's limit or the other's.
But not every tetra is a soft-water fish
This is where the category fails in the other direction. The serpae tetra is recorded from 10–25 dGH — harder water than the molly's own lower bound. Against the molly it produces forced compromises rather than impossibilities, which is a materially different verdict from the cardinal tetra's.
So 'molly plus tetra' has no single answer. It has a range of answers depending on which tetra, and the trade category that groups them conceals exactly the variable that decides it.
The honest recommendation is therefore not a rule about mollies and tetras. It is that the question has to be asked species by species, which is more work and is the only version that is true.
What a forced compromise costs
A forced compromise is not a tank that fails. It is a tank where every inhabitant lives permanently at the edge of its range, and where the usual symptoms — poor colour, short lives, susceptibility to disease after any disturbance — arrive slowly enough that nobody connects them to the water.
Hard water does not kill a cardinal tetra quickly. It shortens its life and dulls it, and the keeper concludes that cardinal tetras are delicate.
What AquariumHQ concludes
Stated as a position rather than as a fact, because that is what it is.
- Treat community as a statement about behaviour and nothing else. Water chemistry is a separate question and it is the one that decides this pairing.
- Against the soft-water characins the molly is a genuine mismatch, and against the hardest-water tetras it is close to workable. Which tetra is not a detail.
- Where a compromise is forced, both animals live at their limits permanently. That is a choice a keeper may make, and it should be made knowingly.
What this still cannot establish
Published rather than omitted. A case study with no stated limits has not had its limits looked for.
- The environmental ranges are FishBase collection records, not tolerance measurements, and a species recorded from a band may do well outside it.
- FishBase's hardness figures are the field this catalogue has most reason to distrust: eight of its hardness values here are identical across four families, which reads as a default. The molly's 11–30 dGH is not one of those, but the tetras' figures deserve the same scrutiny.
- Commercially bred stock of all these species is raised in whatever water the farm has, and captive tolerance is likely broader than the wild records suggest for every animal named here.
- No source consulted quantifies the long-term cost of holding a fish at the edge of its range. That mechanism is reasoned, not measured.