Incompatible5 concerns

Angelfish and Neon tetra

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

VerdictIncompatiblehigh confidence

Should not be attempted. At least one factor is expected to cause harm.

Minimum volume: 270 L
Minimum length: 130 cm
Factors considered: 8

Concerns

blockingpredation

Angelfish reaches about 15 cm and takes fish small enough to swallow. At roughly 3.5 cm, an adult Neon tetra is inside that range. This is usually discovered as fish quietly disappearing overnight rather than as visible aggression.

Derived from: traits.predation.smallFish and adultSize on both records

moderatetemperature

The shared band 24–26 °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

moderatebreeding aggression

Angelfish 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

Angelfish 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

minorzone competition

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 6–7.5.
Hardness requirements overlap at 2–10 °dGH.
Neon 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.

Conditions

Most interesting pairings are conditional on something. These are the things the keeper can actually change.

At least 8 Neon tetra, 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

Documented

Reported in husbandry or veterinary sources rather than derived from stored traits.

AquariumHQ's overall position

This is the clearest life-stage failure in freshwater keeping. AquariumHQ records it as a poor match despite the pairing working, visibly and undeniably, for the first several months.

What would change it: Nothing about the tank. The cardinal tetra is larger-bodied and warmer-water, and is the substitution usually worth making.

Factor by factor

Predation as the angelfish maturesReported
An adult angelfish takes fish that fit in its mouth, and an adult neon tetra does. This is a size relationship rather than a temperament one, and no amount of individual good nature changes a gape.
Life stageReported
The pairing is genuinely peaceful while the angelfish is small, which is why it is attempted so often and why it fails so consistently. Six months of success is the normal course of this pairing, not evidence against the outcome.
TemperatureDerived
Angelfish prefer the warmer end; neon tetras are a cooler-water tetra than their reputation suggests. The overlap is workable but not generous.

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	minor
Life stage	decisive
The individual animal	minor

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	One animal at the edge of its range	The shared band 24–26 °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 6–7.5 pH is usable, though it does not sit centrally for both animals.
Hardness	Comfortable for both	The shared band 2–10 °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.

3 evidence-backed16 derived0 not known

Factor	Basis	Finding
Temperature	Derived from stored traits	One animal at the edge of its range. The shared band 24–26 °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 6–7.5 pH is usable, though it does not sit centrally for both animals.
Hardness	Derived from stored traits	Comfortable for both. The shared band 2–10 °dGH sits centrally in both ranges, with room to drift.
Body size	Derived from stored traits	Angelfish reaches about 15 cm against 3.5 cm for Neon tetra — a ratio of 4.3. Size disparity drives opportunistic predation independently of temperament.
Predation	Evidence-backed	Highest predation risk in either direction is "high". Reported: An adult angelfish takes fish that fit in its mouth, and an adult neon tetra does. This is a size relationship rather than a temperament one, and no amount of individual good nature changes a gape.
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	Neon tetra needs at least 8. 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 200 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: decisive. An adult angelfish takes fish that fit in its mouth, and an adult neon tetra does. This is a size relationship rather than a temperament one, and no amount of individual good nature changes a gape. Reported: The pairing is genuinely peaceful while the angelfish is small, which is why it is attempted so often and why it fails so consistently. Six months of success is the normal course of this pairing, not evidence against the outcome.
Individual variation	Evidence-backed	Individual variation: minor.

Environmental overlap

Parameter	Shared range	Reading
Temperature	24–26 °C	Aim for the middle of the shared band.
pH	6–7.5	Stability matters more than the exact figure.
Hardness	2–10 °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 Angelfish or Neon tetra.
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

AngelfishPterophyllum scalareA tall, laterally compressed cichlid whose height — 20 cm or more from fin tip to fin tip — matters more than its length. It is a cichlid, not a community fish: adults pair off, claim territory and will kill smaller tankmates.Neon tetraParacheirodon innesiThe best-known aquarium fish in the world, and one of the most frequently misjudged. Neons are commercially bred in enormous numbers, which has made them cheap but not hardy — farmed stock is notably less robust than the species' reputation suggests, and losses in the first weeks after purchase are common.