Approx Max Length : 6cm

Northern Mountain Swordtail - Xiphophorus nezahualcoyotl Fish Profile & Care Guide

The Northern Mountain Swordtail (Xiphophorus nezahualcoyotl) is a live-bearing freshwater fish belonging to the family Poeciliidae. It is endemic to Mexico and remains relatively uncommon in the aquarium trade. However, this species is highly regarded for its striking appearance. Males, in particular, can develop exceptionally long and well-defined sword-like extensions on their caudal fins, a characteristic that gives the species its name.

The name 'Nezahualcoyotl' is derived from a revered Aztec ruler and poet, reflecting the cultural significance of this species in its native habitat. Additionally, these fish exhibit intricate patterns on their body and fins, which can vary between individuals, adding to their visual appeal. Due to their rarity in the hobby, they are often sought after by dedicated aquarists who appreciate their unique colouration and natural beauty.

When keeping Northern Mountain Swordtails in an aquarium, it is advisable to maintain a group to promote natural social behaviours and reduce stress. A suitable group size would consist of at least one male and two females, which helps distribute male attention and minimizes potential harassment towards individual females. Given that males can exhibit territorial behaviours, especially during breeding periods, providing ample space and hiding spots is essential.

The Northern Mountain Swordtails are known for their peaceful nature, making them ideal for a community tank. They can be housed with other similarly sized, non-aggressive species, and are particularly well-suited to cohabiting with other livebearers and small, peaceful schooling fish such as Tetras and Rasboras. Additionally, they can thrive alongside bottom-dwelling species like Corydoras Catfish. However, if the aquarium is overcrowded, they may exhibit territorial behaviour or mild aggression toward conspecifics or other livebearers. Providing adequate swimming space and a well-structured environment with sufficient hiding places is essential to minimise stress and conflict.

The Northern Mountain Swordtail thrives in an aquarium with ample swimming space and strategically placed hiding areas. A minimum tank size of 150 litres (approximately 40 gallons) is recommended to accommodate their active nature and occasional territorial behaviour. Incorporating aquatic plants, driftwood, and other decorations will create essential shelter and breeding sites, promoting a sense of security and reducing stress. A filtration system that provides gentle to moderate water flow is ideal, as it mimics their natural riverine habitat while ensuring optimal water quality.

Although not exceptionally vibrant, the Northern Mountain Swordtail possesses a distinct and refined appearance. Its body exhibits a tan to greenish-grey colouration, with each scale outlined in black, giving it a subtly defined texture. A series of six to seven vertical bars adorn the flanks, though their intensity may vary depending on the fish's mood. Some individuals may also display black mottling, further adding to their variability in pattern.

Males are particularly striking, featuring a prominent greenish dorsal fin and an elongated yellow-orange sword on the caudal fin, which is sharply edged in black. In contrast, females have a more subdued appearance, lacking the vertical barring seen in males and exhibiting a generally more uniform colouration. While their patterns may not be as vivid as some other livebearers, the Northern Mountain Swordtail's natural elegance and subtle markings make it a desirable species for aquarists seeking a unique and uncommon addition to their collection.

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Photos

Sexual Dimorphism

The Northern Mountain Swordtail exhibits distinct sexual dimorphism. Males are more vibrantly coloured and possess an elongated caudal fin extension, commonly referred to as a "sword," which is absent in females. Additionally, males feature a gonopodium, a specialized anal fin used for internal fertilization, whereas females retain a standard fan-shaped anal fin. Regarding size, females generally grow larger than males, a characteristic commonly observed within the Xiphophorus genus.

Quick Facts

Scientific Name	Xiphophorus nezahualcoyotl
Year Described	1990
Other Names	Mountain Swordtail, Nezze Sword
Kingdom	Animalia
Classification	Actinopterygii
Order	Cyprinodontiformes
Family	Poeciliidae
Genus	Xiphophorus
Origins	Mexico
Approx Max Size	6 cm
Temperament	Peaceful
Aquarium Level	Middle - Top
Difficulty	Beginner - Intermediate
Shoaling	NA
Best kept as	2 Females per Male
Diet & Feeding	Omnivore
Reproduction	Livebearer
Average Lifespan	Up to 5 Years

Water Parameters

Water Type	Freshwater
pH	7.0 - 8.0
GH	2 - 10
KH	4 - 8

Ideal Temperature Range
℉	72 - 79
℃	22 - 26

Natural Habitat

The Northern Mountain Swordtail is endemic to the Río Tamasi drainage Basin in San Luis Potosí, Mexico, within the Central American region. This species primarily inhabits shallow streams with moderate currents, where the water remains clear, and the substrate consists predominantly of stony or rocky terrain. They are commonly found in areas with algae-covered rocks, serving as a food source and a natural habitat. Unlike many other freshwater environments, these streams typically feature sparse or minimal aquatic vegetation, further distinguishing the species' preferred ecological niche.

Breeding

The Northern Mountain Swordtail exhibits distinct breeding behaviours that are both fascinating and unique. These behaviours are characterized by male courtship displays and mating tactics. Males employ a behaviour known as the "headstand," where they position themselves at an angle, head-down, in front of females to attract their attention. Other courtship behaviours often accompany this display to entice females to mate.

Research indicates that larger males predominantly utilize such courtship displays, while smaller males may resort to coercive mating tactics, such as forced copulation, especially in competitive environments. Females give birth to live young, with broods typically ranging from 10 to 40 fry approximately every 30 days. In captivity, separating the fry from adult fish is advisable, as adults may exhibit cannibalistic behaviour towards their offspring.

Diet & Feeding

The Northern Mountain Swordtail is an omnivorous species that thrives on a varied diet. In captivity, they readily accept high-quality flake or pellet foods as staples. To ensure optimal health and colouration, it is advantageous to supplement their diet with live or frozen foods such as bloodworms, brine shrimp, and daphnia, which provide essential nutrients and mimic their natural feeding habits. Offering a diverse diet meets their nutritional requirements and promotes natural foraging behaviours, contributing to their overall well-being.