(Excerpted from: Extreme Characins  Part 2: Wolves, vampires, and other horrors by Neale Monks)    

Wolf-characins, family Erythrinidae

The wolf-characins, or trahiras, are a small family of very distinctive characins. Unlike the majority of characins, these fish lack an adipose fin. They are also able to breathe air, allowing them to survive in pools and ditches where other fish cannot, and if things get too dire, they are even able to make short excursions across land to find somewhere more hospitable. Another distinctive feature of this group is that the males build a nest and either alone (or, less commonly, with the female) protects the eggs and fry.

The red wolf-characin, Erythrinus erythrinus, is one of the smallest species of its type regularly offered for sale, and arguably the best all-rounder as far as aquarium maintenance goes. The average specimen is pale greenish-pink fish with a dark band running along the midline of the fish and a distinctive shiny spot on the gill covers. There are usually orange markings around the throat, belly and the base of the tail, though these tend to fade on mature specimens. In recent years though, the 'high-fin red' or 'rainbow' variety has become more firmly established in the trade, sometimes under the name Erythrinus sp. 'Peru'. It has a tall dorsal fin marked with reddish-orange. The tail fin is similarly marked, and there are usually additional colour markings on the flanks and sometimes the anal fin as well. Red wolf-characins grow to a maximum size of about 20 cm/8" in the wild, though aquarium specimens tend to be somewhat smaller. Although they are very aggressive towards one another, dissimilar tankmates, such as large barbs and catfish, are generally ignored.

A similar species is the aimara Hoplerythrinus unitaeniatus, distinguished by its larger size (up to 40 cm/16") and the bold horizontal band running along its body. Both these wolf-characins feed primarily on invertebrates in the wild, rather than fish as is popularly assumed, and for optimal health they should be provided things like earthworms, river shrimps, and large aquatic insects. Frozen substitutes are enjoyed as well. Quite different is the trahira Hoplias malabaricus, a fish that feeds on invertebrates when young but as it matures becomes increasingly piscivorous. That said, is adaptable and easily trained to take frozen lancefish and whitebait, though some variation in the form of worms, shrimps, and other invertebrates is probably a very good idea. The main problem with the trahira is its size: at up to 50 cm/20" in length it is only suitable for maintenance in very large aquaria. The combination of large size, snappy disposition, and predatory nature mean that this fish cannot be easily kept with tankmates except perhaps the very largest and most heavily armoured catfish.

Oddly for animals so well adapted to living in difficult environments, the smaller wolf-characins in particular have not proved to be especially hardy. Good water quality, and soft to moderately hard, not too alkaline water seems to be essential. Since these fish like to stay close to cover, a tank that is not too strongly illuminated and planted with Java fern or Anubias attached to bogwood would suit them perfectly.