Incompatible4 concerns

Neon tetra and Oscar

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: 610 L
Minimum length: 180 cm
Factors considered: 8

Concerns

blockingpredation

Oscar reaches about 40 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

minorterritory

Oscar 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

minorbreeding aggression

Oscar 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

Why it can work

pH requirements overlap at 6–7.5.
Hardness requirements overlap at 3–10 °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.
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 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

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

Incompatible. Peaceful only while the oscar is too small to have grown into what it is, and the interval varies with feeding rather than with temperament.

What would change it: Nothing available to a keeper. The variable is the oscar reaching a size at which a 3 cm tetra registers as food, and that is growth rather than behaviour — it cannot be trained out, fed around or housed away in any tank that holds both. This is one of the few pairings where the honest answer is that time settles it and the keeper does not.

Factor by factor

GapeReported
An adult oscar eats fish of this size as a matter of course. There is no tank size, cover arrangement or feeding regime that changes it.
Bioload and spaceDerived
The oscar's minimum footprint is several times the tetra's, so the pairing is constrained long before behaviour is considered.

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	none
Life stage	decisive
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.

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	Workable	The shared band 3–10 °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.

2 evidence-backed17 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	Workable. The shared band 3–10 °dGH is usable, though it does not sit centrally for both animals.
Body size	Derived from stored traits	Oscar reaches about 40 cm against 3.5 cm for Neon tetra — a ratio of 11.4. 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 "strong".
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	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	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 "near-certain".
Effect on plants	Derived from stored traits	Oscar 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 450 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	Evidence-backed	Life-stage dependency: decisive. See the documented record.
Individual variation	Evidence-backed	Individual variation: none.

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	3–10 °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 Neon tetra or Oscar.
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

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.OscarAstronotus ocellatusA 35 cm cichlid with genuine individual character and an appetite to match. Sold at 5 cm into tanks that will be inadequate within a year, and one of the most commonly rehomed fish in the hobby.