Incompatible3 concerns

Goldfish 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: 400 L
Minimum length: 150 cm
Factors considered: 6

Concerns

blockingpredation

Goldfish reaches about 35 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

majortemperature

The shared band 20–22 °C sits at the top of one animal's range and the bottom of the other's. Both would be kept at their limits to accommodate the other. Both animals would be kept at the edge of their ranges to accommodate the other. Nothing here is a recommendation; it is the arithmetic of two incompatible requirements meeting at a point.

Derived from: environment.temperature range position on both records

moderateph

pH ranges overlap only across 7–7.5, which is tighter than most tap-water supplies hold naturally.

Derived from: environment.ph overlap width

Why it can work

Hardness requirements overlap at 8–10 °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

Documented

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

AquariumHQ's overall position

Incompatible, and the reason is thermal rather than behavioural. This is the pairing where the derived assessment and the documented evidence agree most completely, and where 'it worked for me' most often means one of the two animals was quietly being shortened.

Factor by factor

TemperatureReported
The central problem, and it is not behavioural. A goldfish kept at tropical community temperatures has a shortened life; a neon tetra kept at goldfish temperatures is at the bottom of its range. There is no temperature at which both are correctly kept.
Predation as the goldfish growsReported
An adult goldfish is large enough to take a neon tetra, and goldfish are opportunistic feeders.
BioloadDerived
Goldfish produce substantially more waste than a community fish of comparable length, which is why goldfish stocking advice reads oddly to a tropical keeper.

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	significant
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	Forced compromise — neither animal is where it wants to be	The shared band 20–22 °C sits at the top of one animal's range and the bottom of the other's. Both would be kept at their limits to accommodate the other.
pH	Forced compromise — neither animal is where it wants to be	The shared band 7–7.5 pH sits at the bottom of one animal's range and the top of the other's. Both would be kept at their limits to accommodate the other.
Hardness	Forced compromise — neither animal is where it wants to be	The shared band 8–10 °dGH sits at the bottom of one animal's range and the top of the other's. Both would be kept at their limits to accommodate the other.

Forced compromise — neither animal is where it wants to be

Both animals would be kept at the edge of their ranges to accommodate the other. Nothing here is a recommendation; it is the arithmetic of two incompatible requirements meeting at a point.

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.

4 evidence-backed15 derived0 not known

Factor	Basis	Finding
Temperature	Evidence-backed	Forced compromise — neither animal is where it wants to be. The shared band 20–22 °C sits at the top of one animal's range and the bottom of the other's. Both would be kept at their limits to accommodate the other. Reported: The central problem, and it is not behavioural. A goldfish kept at tropical community temperatures has a shortened life; a neon tetra kept at goldfish temperatures is at the bottom of its range. There is no temperature at which both are correctly kept.
pH	Derived from stored traits	Forced compromise — neither animal is where it wants to be. The shared band 7–7.5 pH sits at the bottom of one animal's range and the top of the other's. Both would be kept at their limits to accommodate the other.
Hardness	Derived from stored traits	Forced compromise — neither animal is where it wants to be. The shared band 8–10 °dGH sits at the bottom of one animal's range and the top of the other's. Both would be kept at their limits to accommodate the other.
Body size	Derived from stored traits	Goldfish reaches about 35 cm against 3.5 cm for Neon tetra — a ratio of 10.0. Size disparity drives opportunistic predation independently of temperament.
Predation	Evidence-backed	Highest predation risk in either direction is "moderate". Reported: An adult goldfish is large enough to take a neon tetra, and goldfish are opportunistic feeders.
Territoriality	Derived from stored traits	Neither animal holds a territory.
Breeding aggression	Derived from stored traits	Neither becomes notably more aggressive while spawning.
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 "none". 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	Goldfish eats 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	Evidence-backed	Life-stage dependency: significant. See the documented record.
Individual variation	Evidence-backed	Individual variation: none.

Environmental overlap

Parameter	Shared range	Reading
Temperature	20–22 °C	Aim for the middle of the shared band.
pH	7–7.5	Stability matters more than the exact figure.
Hardness	8–10 °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 Goldfish 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

GoldfishCarassius auratusA large, long-lived, cold-water pond fish that has spent a century being sold as a bowl ornament. Single-tail goldfish reach 25–30 cm, live for decades, produce an enormous waste load and cannot be kept properly in an indoor tank of ordinary size.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.