Central American River Biotope: Cichlids and Livebearers in Hard Water

Central American river biotope: warm, mineral-rich, neutral-to-alkaline water, rocky streams versus slow sandy rivers and cenotes, cichlids and livebearers, and aggression control.

Rivers of the Central American land bridge

Between southern Mexico and Panama, rivers drain to both the Atlantic (Caribbean) and the Pacific slope. The largest, the Usumacinta, runs about 1,000 km through Guatemala and the Mexican states of Chiapas and Tabasco to the Gulf of Mexico, forming the Pantanos de Centla wetlands of 302,702 ha at its confluence with the Grijalva (Wikipedia). On the Yucatán Peninsula surface rivers are replaced by more than 3,000 water-filled limestone sinkholes, the cenotes, of which about 1,400 have been registered (Practical Fishkeeping). FishBase places the firemouth cichlid Thorichthys meeki in the Usumacinta and Belize River drainages, and the convict cichlid Amatitlania nigrofasciata on both the Pacific and Atlantic slopes from El Salvador through Guatemala and Honduras.

These waters are typically warm, mineral-rich and neutral to alkaline. Wikipedia reports measurements from four convict cichlid habitats in Costa Rica: pH 6.6–7.8, carbonate hardness 63–77 ppm CaCO₃ and daily temperatures of 26–29 °C. FishBase gives pH 7.0–8.0 and 9–20 dH for A. nigrofasciata, pH 7.0–8.0 and 9–19 dH for the green swordtail Xiphophorus hellerii, and pH 7.5–8.2 and 11–30 dH for the molly Poecilia sphenops. Cenote water owes its turquoise colour to dissolved limestone (Practical Fishkeeping). FishBase lists T. meeki at a lower pH 6.5–7.5 and 0–10 dH, so hardness varies by drainage and a single "Central American" value does not exist.

Flow, substrate and hardscape

Habitat descriptions differ by species and this shapes the aquarium. A. nigrofasciata prefers moving water and is most often found among rocks and sunken branches; FishBase describes it from creeks and streams to the shallows of large, fast-flowing rivers, sheltering in crevices and root systems, in volcanic lakes up to 1,500 m elevation (Wikipedia). X. hellerii lives in rapidly flowing, densely vegetated waterways, warm springs, weedy canals and ponds (FishBase). By contrast T. meeki favours slow-moving lower and middle river sections, mud- and sand-bottomed canals and rocky ponds near shoreline vegetation (FishBase), over sand, mud, marlstone or travertine limestone, among Myriophyllum and Nymphaea (Seriously Fish). The Jack Dempsey Rocio octofasciata occupies swampy, warm, murky water and weedy ditches from southern Mexico to Honduras (Wikipedia).

Substrate: fine sand, or fine gravel for firemouths, because Thorichthys and many other cichlids sift and dig (Seriously Fish, Practical Fishkeeping).
Hardscape: rounded rocks and crevices for rheophilic species such as A. nigrofasciata; large pieces of driftwood and protruding roots, which also divide territories (FishBase, Practical Fishkeeping).
Plants: rooted plants are usually uprooted by digging cichlids; Practical Fishkeeping suggests Anubias attached to wood and floating plants for shade, and its cenote layout used mats of Riccia fluitans in place of the native Chara.
Flow: a rocky-stream layout for convicts and swordtails carries noticeable current, while a firemouth or cenote layout is calm.

Typical fauna

The region's freshwater fish community is dominated by two families: heroine cichlids and poeciliid livebearers. FishBase and Seriously Fish describe the cichlids named below as substrate spawners with biparental care, in which pairs defend a territory and both parents guard eggs and fry. Livebearers occupy the water column and shallows above them.

Amatitlania nigrofasciata (convict cichlid) — Guatemala to Panama; to 10 cm SL; omnivore; serially monogamous, cave or crevice spawner, eggs hatch in about 72 hours; IUCN Data Deficient (Wikipedia, FishBase).
Thorichthys meeki (firemouth cichlid) — Mexico, Belize and Guatemala; to 17 cm TL; omnivore that sifts substrate for small crustaceans and detritus; 100–500 eggs on open substrate; IUCN Least Concern, 2018 (FishBase, Seriously Fish).
Rocio octofasciata (Jack Dempsey) — southern Mexico to Honduras; males to about 25 cm; carnivore; 500–800 eggs on a flat hard surface (Wikipedia).
Herichthys cyanoguttatus (Texas cichlid) — lower Rio Grande drainage of Texas and north-eastern Mexico, the northern limit of the group; to about 30 cm; dH 8–25 (Tropical Fish Hobbyist).
Xiphophorus hellerii (green swordtail) — Río Nantla, Veracruz, to north-western Honduras; males 14 cm TL, females 16 cm TL; IUCN Least Concern (FishBase).
Poecilia sphenops (molly) — Mexico to Colombia; to 7.5 cm SL; grazes green algae; IUCN Least Concern (FishBase). Practical Fishkeeping's cenote-style layout was stocked with the livebearer Poecilia cf. salvatoris, an active algae grazer.

Aggression management

Central American cichlids are territorial, and aggression peaks during breeding. Seriously Fish calls A. nigrofasciata "not a good community fish" and advises a single pair in a tank with a base of at least 90 × 30 cm, with rocks, bogwood, roots and caves. Wikipedia adds that aggression in this species increases at 30 °C compared with 26 °C, so keeping temperature in the lower part of the range is a management tool in itself. Practical Fishkeeping warns that in tanks under 80 cm the territorial displays of firemouths, with flared red throat and spread fins, can endanger most tankmates, and describes subordinate fish signalling submission by tilting head-down and clamping their fins.

Space and structure are the main levers. Seriously Fish gives a 120 × 30 cm base as the minimum for a firemouth pair, with driftwood and shaded zones creating separate territories; Tropical Fish Hobbyist requires about 470 L (125 gal) before a Texas cichlid is combined with other large, robust Central American cichlids. Seriously Fish rates T. meeki as not especially aggressive and compatible with similar-sized Central American cichlids and livebearers, whereas Practical Fishkeeping cautions against mixed cichlid tanks around firemouths. R. octofasciata, according to Wikipedia, becomes more territorial when kept with only a few other fish and settles better in a well-populated tank with many hiding places.

Livebearers complete the biotope and occupy the upper water. Practical Fishkeeping lists mollies and other livebearers among suitable companions for firemouths, and FishBase notes that male X. hellerii are themselves territorial toward one another. Wikipedia notes that the constant digging of convict cichlids calls for robust filtration; Practical Fishkeeping's cenote layout used a weekly one-third water change.