Cardinal tetra and Discus
Can these two share a tank? The assessment below is derived from stored behavioural and environmental traits, not from a compatibility chart.
Should not be attempted. At least one factor is expected to cause harm.
- Minimum volume
- 475 L
- Minimum length
- 150 cm
- Factors considered
- 8
Concerns
Discus reaches about 20 cm and takes fish small enough to swallow. At roughly 5 cm, an adult Cardinal tetra is inside that range. This is usually discovered as fish quietly disappearing overnight rather than as visible aggression.
Derived from: traits.predation.smallFish, adultSize on both records, and the stored body form and gape of both
The shared band 28–30 °C sits at the top of one animal's range, so that animal is permanently at its limit even though the overlap looks generous. The overlap sits at the extreme of at least one animal's range, so that animal is permanently at its limit. A non-empty overlap is not the same as a suitable one.
Derived from: environment.temperature range position on both records
Discus 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
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 5.5–7.
- Hardness requirements overlap at 1–6 °dGH.
- Cardinal tetra 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.
- Discus needs a group of at least 5. 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 8 Cardinal tetra, counted as one group
- At least 5 Discus, 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
The classic discus community pairing, and it works because the cardinal tetra is unusually heat-tolerant rather than because 30 °C is where a cardinal wants to live. Lifespan data for tetras held at the top of their range is not something AquariumHQ holds, and the absence is worth stating.
What would change it: Running the discus at 28 rather than 30 brings both animals into a more comfortable band, at some cost to the discus's growth rate.
Factor by factor
- TemperatureDerived
Discus are advised at 28–31 °C and cardinal tetras at 24–30. The overlap is real — two degrees — and it is the top of the tetra's range held continuously, which is a different demand from encountering it.
- The traditional pairingReported
Cardinals are the standard discus dither fish and have been for decades, specifically because they are the tetra that tolerates the heat. That is a real, consistent report and it is also the reason the pairing is used rather than evidence that it is comfortable.
- Feeding competitionReported
Discus feed slowly and deliberately; cardinals do not. In a tank fed for discus the tetras take a disproportionate share, which is manageable and routinely underestimated.
| Depends on | How much |
|---|---|
| Tank size and dimensions | significant |
| Life stage | none |
| The individual animal | none |
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 | One animal at the edge of its range | The shared band 28–30 °C sits at the top of one animal's range, so that animal is permanently at its limit even though the overlap looks generous. |
| pH | Workable | The shared band 5.5–7 pH is usable, though it does not sit centrally for both animals. |
| Hardness | Comfortable for both | The shared band 1–6 °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 | One animal at the edge of its range. The shared band 28–30 °C sits at the top of one animal's range, so that animal is permanently at its limit even though the overlap looks generous. |
| pH | Derived from stored traits | Workable. The shared band 5.5–7 pH is usable, though it does not sit centrally for both animals. |
| Hardness | Derived from stored traits | Comfortable for both. The shared band 1–6 °dGH sits centrally in both ranges, with room to drift. |
| Body size | Derived from stored traits | Discus reaches about 20 cm against 5 cm for Cardinal tetra — a ratio of 4.0. Size disparity drives opportunistic predation independently of temperament. |
| Predation | Derived from stored traits | Highest predation risk in either direction is "moderate". |
| Territoriality | Derived from stored traits | Strongest territoriality across the pair is "mild". |
| Breeding aggression | Derived from stored traits | Breeding aggression reaches "moderate". 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 | Cardinal tetra needs at least 8; Discus needs at least 5. 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 | 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 "near-certain". |
| 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 350 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 | Evidence-backed | Life-stage dependency: none. See the documented record. |
| Individual variation | Evidence-backed | Individual variation: none. |
Environmental overlap
| Parameter | Shared range | Reading |
|---|---|---|
| Temperature | 28–30 °C | Aim for the middle of the shared band. |
| pH | 5.5–7 | Stability matters more than the exact figure. |
| Hardness | 1–6 °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 Cardinal tetra or Discus.
- 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.
Cardinal tetra
Blackwater
the Rio Negro, true blackwater
8–40 µS/cm · low flow · moderate oxygen
Discus
Soft-water lake
warm Amazon floodplain lakes and slow blackwater
10–80 µS/cm · still flow · moderate oxygen
- TemperatureCardinal tetra 24–30°CDiscus 28–31°C
Both inside their range from 28–30°C — 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.
- pHCardinal tetra 4–7Discus 5.5–7.2
Both inside their range from 5.5–7 — 88% 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 hardnessCardinal tetra 1–6 dGHDiscus 1–8 dGH
Both inside their range from 1–6 dGH — 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.
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.