Acarichthys heckelii

The Threadfin Acara is a beautiful and graceful cichlid from South America, easily recognized by the long, thread-like extensions on its dorsal fin that give it its name. These fish typically have a silvery-tan body with an attractive pearly sheen and a distinct dark spot on their side. They are moderately active and spend most of their time exploring the bottom and middle levels of the aquarium.

To keep them happy, a large aquarium is essential, as they need plenty of space to swim. A soft, sandy bottom is a must, as Threadfin Acaras love to dig and sift through the sand in search of food. This digging behavior means you should choose hardy plants like Java Fern or Anubias and attach them to rocks or driftwood, or pot them securely. They are social fish and should be kept in a small group of at least five individuals. This helps them feel more secure and reduces any bickering. While generally peaceful for a cichlid, they can become territorial, especially when they are ready to mate. They thrive in warm, soft, and slightly acidic water with a gentle current. They are not picky eaters and will readily accept a variety of foods, including sinking pellets, flakes, and frozen options.

The Threadfin Acara, Acarichthys heckelii, is a fascinating geophagine cichlid hailing from the slow-moving blackwater rivers and tributaries of the Amazon basin in South America. Its natural habitat dictates its ideal aquarium conditions: warm, soft, acidic water, often stained with tannins from decaying leaves and wood. To truly see this fish flourish, hobbyists should aim to replicate this biotope. A spacious tank with a deep, soft sand substrate is non-negotiable. This allows the Acara to engage in its natural and constant sand-sifting behavior, where it takes mouthfuls of sand to filter out food particles. This digging is why they are often labeled 'plant rooters'; they will uproot most conventionally planted flora. Therefore, aquascaping should focus on robust, epiphytic plants like Anubias, Java Fern, and Bolbitis attached to extensive driftwood structures. Floating plants are also beneficial for diffusing light and enhancing the fish's sense of security.

Water movement should be gentle, replicating the slow currents of their native environment. While filtration must be robust to handle their moderate waste output, the filter outflow should be baffled or directed towards the glass to avoid creating strong currents. A group of at least five is crucial for their well-being. Keeping them singly can lead to extreme shyness or, conversely, un-checked aggression towards tank mates. In a group, a natural social hierarchy is established, and disputes are typically minor. Suitable tank mates include other peaceful, similarly-sized South American fish like larger tetras (e.g., Bleeding Heart Tetras), severums, and some catfish species. Avoid housing them with small, timid fish or overly aggressive cichlids.

Their omnivorous diet in the wild consists of small invertebrates and detritus found in the substrate. In captivity, a high-quality sinking cichlid pellet should form the staple of their diet. This must be supplemented with a variety of frozen and live foods such as bloodworms, mysis shrimp, brine shrimp, and daphnia to ensure a balanced nutritional intake. Breeding is considered challenging. They are substrate spawners that create and defend a territory, often digging a cave under a piece of wood or rock where they lay their eggs. Achieving a spawn requires pristine water conditions, a well-conditioned and compatible pair, and specific environmental triggers that are not always easy to replicate.

Acarichthys heckelii, the sole described species within its genus, is a member of the Cichlidae family, positioned within the Geophaginae subfamily. This taxonomic placement correctly implies its ethological leaning towards geophagy, or 'earth-eating,' a foraging behavior characterized by the ingestion and sifting of substrate. Native to the Amazon, Orinoco, and Essequibo river basins in South America, it primarily inhabits lentic, blackwater environments with soft, acidic water chemistry (low pH and water hardness).

Morphologically, A. heckelii is distinguished by a moderately compressed, ovoid body and, most notably, the filamentous extensions of the anterior soft rays of the dorsal fin, which are more pronounced in sexually mature males. This feature is a key component of its common name, the Threadfin Acara. A conspicuous dark, mid-lateral blotch is present, a common marking among cichlids used for intraspecific communication.

Its behavior is complex and deeply rooted in its ecology. The substrate-sifting is not random but a targeted foraging strategy to uncover benthic invertebrates and organic detritus, forming the bulk of its omnivorous diet. The species is gregarious, displaying a clear requirement for social grouping. This shoaling behavior likely serves as an anti-predator defense and increases foraging efficiency. Within the group, a social hierarchy is formed, mitigating overt aggression which otherwise becomes pronounced when kept in insufficient numbers or in cramped conditions. Territoriality is primarily expressed during reproduction. As a biparental substrate spawner, a pair will select and defend a site, typically a self-excavated cave or tunnel beneath hardscape, for oviposition. The 'difficult' breeding classification in aquaria stems from the need to replicate specific environmental cues (e.g., changes in water chemistry or temperature simulating seasonal shifts) and the prerequisite of a bonded pair forming from a larger social group. The IUCN status of 'Not Evaluated' (NE) signifies a data deficiency in formal population assessment, though the species is not considered threatened due to its widespread distribution.