Cardinal tetra and Electric blue hap
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
- 540 L
- Minimum length
- 180 cm
- Factors considered
- 9
Concerns
Electric blue hap 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
Reported pH ranges do not overlap: 4–7 against 7.6–8.6. Chasing a compromise pH with additives is less stable than keeping each animal in water that suits it.
Derived from: environment.ph on both records
Hardness requirements do not overlap: 1–6 °dGH against 10–20 °dGH. Soft-water and hard-water animals kept together mean one of them is permanently compromised, usually in ways that only show up as shortened lifespan.
Derived from: environment.generalHardness on both records
Electric blue hap 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
Electric blue hap 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
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
- 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.
- Electric blue hap needs a group of at least 3. 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 3 Electric blue hap, 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
No behavioural evidence held
AquariumHQ holds no behavioural evidence for this pairing. The verdict is derived entirely from stored traits — it is arithmetic on our own records, not a report about how the pairing actually goes.
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 | No overlap at all | Ranges do not meet: 4–7 against 7.6–8.6 pH. |
| Hardness | No overlap at all | Ranges do not meet: 1–6 against 10–20 °dGH. |
No overlap at all
The ranges do not meet. One animal would be kept outside its range permanently.
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 | No overlap at all. Ranges do not meet: 4–7 against 7.6–8.6 pH. |
| Hardness | Derived from stored traits | No overlap at all. Ranges do not meet: 1–6 against 10–20 °dGH. |
| Body size | Derived from stored traits | Electric blue hap 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 "near-certain". |
| 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 | 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; Electric blue hap needs at least 3. 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 | Electric blue hap uproots plants, which constrains the aquascape rather than the pairing. |
| Tank dimensions | Derived from stored traits | The more demanding of the two needs at least 400 L and 150 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 | Not known | AquariumHQ holds no record of whether this pairing changes as either animal matures. Stored traits describe adults, so a pairing that fails at adult size looks identical to one that never fails. |
| Individual variation | Not known | AquariumHQ holds no record of how much this pairing varies between individuals. For several species — bettas and cichlids especially — that variation decides the outcome and a species-level verdict cannot express it. |
Environmental overlap
| Parameter | Shared range | Reading |
|---|---|---|
| Temperature | 24–27 °C | Aim for the middle of the shared band. |
| pH | No overlap | Chasing a compromise pH is less stable than keeping each animal separately. |
| Hardness | No overlap | Soft-water and hard-water animals together means one is permanently compromised. |
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 Electric blue hap.
- 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.
No overlap. There is no ph at which both species are inside their advised range.
Hardness (°dGH)
Scale: 0 to 30 °dGH.
No overlap. There is no hardness at which both species are inside their advised range.
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
Electric blue hap
Lake Malawi
200–280 µS/cm · low flow · near saturation oxygen
- TemperatureCardinal tetra 24–30°CElectric blue hap 24–27°C
Both inside their range from 24–27°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.
- pHCardinal tetra 4–7Electric blue hap 7.6–8.6
No overlap.
No overlap at all. There is no temperature, pH or hardness that suits both.
- General hardnessCardinal tetra 1–6 dGHElectric blue hap 10–20 dGH
No overlap.
No overlap at all. There is no temperature, pH or hardness that suits both.
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.