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Predator and prey size

Size disparity is the single most predictive factor in freshwater compatibility and the one most often argued away. The argument usually takes the form 'mine has never bothered anyone', which is true, and which describes a fish that has not yet reached the size at which the question arises.

Short answer

A fish eats what fits in its mouth. Temperament decides how often it tries; gape decides whether it can succeed, and gape is a fact about anatomy that no amount of good nature changes.

Why this is a size relationship rather than a temperament one

A peaceful fish and an aggressive fish of the same size pose different risks to a tankmate they cannot swallow. Neither poses a different risk to one they can. Predation is not aggression: it is feeding, and a completely placid animal feeds.

This is why the angelfish and neon tetra pairing fails so reliably and so late. The angelfish is not becoming aggressive. It is growing a mouth, and at some point that mouth is larger than a neon tetra. Six months of peaceful cohabitation is the normal course of that pairing rather than evidence against the outcome.

A predator's mouth compared with prey of three sizes at two life stagesTwo rows. In the upper row a juvenile predator of five centimetres has a small mouth, and a three-centimetre tetra beside it does not fit. In the lower row the same species at fifteen centimetres has a mouth roughly three times larger, and the same three-centimetre tetra now fits inside it. The caption notes that nothing about the predator's temperament changed between the two rows.Same fish, same temperament, twelve months apartAt 5 cm3 cm tetralarger than the gape — safeAt 15 cmthe same 3 cm tetranow inside the gapeNothing in the fish's behaviour changed. Its mouth did.
Why a peaceful fish still eats its tankmates. The mouth is the constraint, and it grows with the fish. Source: AquariumHQ predation traits and adult-size data

The rough arithmetic, and its limits

As a first approximation, a fish can swallow prey up to roughly a third of its own length, and considerably more where the prey is slender. That figure is a starting point for asking the question rather than a threshold to design to.

  • Body shape matters as much as length. A slender fish of 4 cm and a deep-bodied fish of 4 cm are not equally swallowable, which is why the cardinal tetra survives situations the neon does not.
  • Some fish do not need to swallow. Fin-nippers, and the leaf fish's protrusible jaws, and a snakehead's willingness to take a bite out of something larger, all fall outside the arithmetic.
  • Nocturnal predators eat sleeping fish. A tankmate that is fast enough during the day is not fast enough at three in the morning, which is how featherfin squeakers and spiny eels quietly reduce a community tank.
  • Adult size is the input, not current size. Almost every failure of this rule is a keeper doing the arithmetic with the fish in front of them.

How AquariumHQ stores it

Predation is recorded per record as five separate graded risks rather than as a single number: small fish, adult dwarf shrimp, shrimplets, snails, and harassment of sedentary tankmates. They are separated because they come apart in practice. A pea puffer is a near-certain risk to snails and a moderate one to small fish. A cuckoo catfish is the reverse.

The compatibility engine reads those fields alongside adult size, and reports size disparity as its own factor with its own reasoning, so a verdict driven by gape says so rather than presenting itself as a judgement about temperament.

The species where this decides the answer

  • Anything sold small that reaches over 30 cm: the redtail catfish, the giant snakehead, the silver arowana, the clown knifefish, the iridescent shark.
  • Mid-sized cichlids sold at 3 cm — the jack dempsey, the green terror, the jaguar cichlid — where the community tank works for a year.
  • Nocturnal bottom-dwellers that are peaceful by day: bichirs, spiny eels, larger Synodontis.
  • Any tank containing dwarf shrimp, where the list of fish that reliably ignore adult shrimp is much shorter than the list that ignores them most of the time.