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Longhorn Cowfish

Lactoria cornuta

Marine Marine Indo-Pacific

An unmistakable boxy fish sporting horn-like protrusions on its brow. Although charismatic and slow-moving, it grows remarkably large and needs gentle water flow. Crucially, if stressed or dying, it can release deadly ostracitoxin into the water, which can wipe out every tank mate in minutes.

Also called: Longhorn cowfish, Boxfish, Cofferfish, Cowfish.

35 cm adult length, drawn to scale. round body.

Outgrows a typical home tank. Check this fish against the adult size above before buying a juvenile. The minimum shown is the least it is kept in for good, not a comfortable size.
Saltwater species. The planner judges it with a marine rule set. A reef or fish-only tank takes months to mature and costs more to run than freshwater. See the marine page.

Try it in the planner Opens a tank sized to its minimum with 1 fish added.

What it needs

ItemValueBecause
Adult length 35 cm Used for the Space check: the tank must be long enough for the fish to turn round.
Smallest tank 700 L, at least 180 cm long estimated The least water and floor length this species is kept in for good, as a single fish or smallest group.
Extra water per added fish 300 L estimated What each further fish adds once the minimum is met.
Waste load Class 5 of 5 estimated How much waste an adult makes against its size. Feeds the Load check.
Temperature 23 to 27 C Stock sharing a tank need overlapping ranges.
pH 8.1 to 8.4 Compared with tankmates in the Water check.
Hardness GH 8 to 12, KH 8 to 12 Degrees of general and carbonate hardness.
Flow Low Low-flow fish are stressed in a strong current.
Light Moderate Matched to the lighting class on the needs sheet.
Group size Single fish is fine Not a schooling species.
Temperament Peaceful Not territorial.
Swims in Middle water Fish that use different levels share a tank more easily.
Lifespan About 8 years A fish is a long commitment.
Experience Experienced keepers only A guide to how forgiving the species is of mistakes.
Salinity Marine, about SG 1.023 to 1.025 Needs salt water, a different build and a longer-running tank than a freshwater set-up.

Tags marked estimated are judged from the size and habits of the species rather than measured, so treat them as a sensible default.

Diet and tank layout

Longhorn Cowfish is an carnivore that swims in the middle of the tank. It is safe with plants. Give it some cover.

Tankmates

Works with

Depends

Avoid

Compare any two fish on the compatibility page, or print the equipment for a stock on the needs sheet. How this is worked out: the method.