The Harlequin (Trigonostigma heteromorpha) has been a long-standing favourite in the aquarium hobby. Their popularity is derived from their ability to tolerate a wide range of water conditions, combined with their active nature, small size and ability to integrate easily into the community aquarium.
Harlequins are found across Indonesia, Sumatra and Thailand, where they are found in acidic but not deeply blackened streams, and sometimes peaty swamps. These streams tend to have a yellowed tinge; a feature which can be replicated in the aquarium, via the addition of driftwood, if the hobbyist desires.
In order to replicate a natural environment, ample driftwood and a fine sand substrate should be combined with plenty of live plants. These fish prefer softer, acidic waters and although they can cope in a neutral or slightly alkaline community tank, it’s worth remembering that some pathogens fare better in alkaline waters. Aim for a pH of between 5.0 and 7.0, with a hardness of below 12°DH. Set the temperature somewhere between 22-28°C to replicate natural conditions. Water movement in the aquarium should be limited, as Harlequins do not thrive in high, rushing water flows and may actively avoid areas in the tank where there is too much flow.
Harlequins do well in heavily planted aquariums and providing thick beds of Cryptocoryne and plenty of Java fern will go towards replicating their natural environment. Using low growing plants will provide all the benefits of a heavily planted aquarium, while still allowing plenty of swimming space. The addition of floating plants will provide extra cover and cause the fish to become more confident in their behaviour.
Although Harlequins are small, only growing to 5cm in length, they are schooling fish which live in large groups in the wild. Therefore, it is essential to purchase no less than 12 individuals in order to ensure longevity and prevent shy, reclusive behaviour. Harlequins should be kept in an aquarium no smaller than 60cm long.
Harlequins are a very peaceful species and can be combined with almost all other similar sized species. However, they should never be combined with large aggressive species such as South American cichlids, unless the tank is large enough to accommodate all fish without the fish being forced to constantly interact. If geographical correctness is a concern, species of the genus Acanthopthalmus, Puntius and Sphaerichthys are suitable considerations.
Note from Author
This is a species that, for a long time, I was unable to keep due to my aquariums containing species which were not suitable co-inhabitants. When I eventually bought a group of Harlequins, I instantly fell in love. Their active nature, combined with their subtle yet beautiful colouration, made them a simple, yet effective addition to the aquarium. Harlequins will definitely remain one of my favourite schooling fish well into the future.