Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Trigonostigma heteromorpha


Harlequin, Trigonostigma heteromorphaHarlequin, Trigonostigma heteromorpha
Photos by Andy Rapson


Trigonostigma: Greek, tres, tria = three + Greek, gonias= angle, corner + Greek stigma = signal.

General Notes:

Harlequins are great community fish with tank mates which require the same conditions. They do require slightly acidic and soft water in order to thrive, even after many generations of captive breeding they won't live for their full natural life expectancy in hard alkaline water. Harlequins are very peaceful and needs to be kept with other peaceful fishes. Keep in a group rather than singly or in pairs, harlequins need the security of a shoal of their own kind. Harlequins will often be seen circling each other with their fins erect, this is just harmless ritualised combat usually to determine the pecking order. It has nothing to do with courtship and neither fish will come to any harm.


Wild harlequins eat small worms, crustaceans and insects. In captivity they are relatively unfussy and will take most of the food offered. Their diet should include some live or frozen food.


Males have more vivid colouration and are smaller and slimmer than the females when adult.


Harlequins breed at very low pH levels and in very soft water. The fry can be slowly acclimatised to less extreme conditions which are commonly found in most aquariums but there are limits. They will not thrive for long in hard or alkaline water.
When breeding they carefully place each individual egg on the underside of a leaf, both parents have to hang up side down in order to do this.

Wild status

Wild populations face no immediate threats, in some parts of their range in the Thai region local populations have been impacted on but the fish is wide spread and most populations are thriving.

Information at a glance

pH: 5 - 7
dGH: 5 - 12
Temperature: 22 - 28°C (72 - 82°F)
Diet: Omnivore
Size: 5cm (2in)
Min tank size: 60 litres
Difficulty level: Intermediate
Aquarium type: Community
Swimming level: All levels

Distribution and habitat

distribution map for Harlequin, Trigonostigma heteromorpha

Origin: Asia: Thailand (Narathiwat southwards) to Sumatra, Indonesia

Habitat: Found in small forested streams, typically with moderate current and soft, acidic, water


Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Cypriniformes
Family: Cyprinidae
Genus: Trigonostigma
Species: T. heteromorpha

Common name:

Synonyms: Rasbora heteromorpha