A beautiful and highly active livebearer endemic to the fast-flowing mountain streams of the Río Pánuco basin in Mexico. Known affectionately in the hobby as 'Nezzies', they prefer slightly cooler, highly oxygenated water with moderate to high flow compared to standard tropicals. As livebearers, they are prolific breeders. They are strong swimmers and known jumpers, requiring a secure lid.
Northern Mountain Swordtail At a Glance
Northern Mountain Swordtail Tank Requirements
Will Northern Mountain Swordtail fit in your tank?
Care guides tell you what Northern Mountain Swordtail needs, the next question is whether your tank can safely provide it. Test Northern Mountain Swordtail against your tank size, maintenance needs, and the fish you already keep before you buy.
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Northern Mountain Swordtail Water Parameters
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Northern Mountain Swordtail Temperament & Compatibility
Northern Mountain Swordtail Varieties
Each variety inherits the species defaults above. Only differences are highlighted.
Male
Males develop a distinctive yellow and black upturned 'sword' extending from the lower tail lobe, and a modified anal fin (gonopodium). They are very active, constantly pursuing females and sparring with other males for dominance. It is best to maintain a ratio of 1 male to 2-3 females to disperse harassment.
Differs in 3 traits from base species
Also known as: Swordtail Male, Sword
Female
Females are larger, deeper-bodied, lack the tail sword, and have a standard fan-shaped anal fin. They are generally peaceful but require dense planting and space to escape the constant attention of males. They will drop live fry roughly every 30 days.
Differs in 2 traits from base species
Also known as: Swordtail Female, Pregnant
Northern Mountain Swordtail Care Guide Summary
The Northern Mountain Swordtail, scientifically known as Xiphophorus nezahualcoyotl, belongs to the Livebearers family group. Adults reach a maximum size of approximately 5.5 cm and have a fusiform body shape with a fast growth rate. Classified as mostly peaceful, this species primarily occupies the surface and mid-level zones of the aquarium and produces a high level of biological waste.
To thrive, the Northern Mountain Swordtail requires a freshwater only aquarium with a minimum volume of 110 litres and a tank length of at least 90 cm. Water temperature should be maintained between 20°C and 25°C, with a pH range of 7–8 and a general hardness of 8–20 dGH. A high water flow is preferred.
The Northern Mountain Swordtail is a rewarding species to keep, provided its core environmental needs are met. This species is available in 2 varieties: Male and Female. Each may differ slightly in size, temperament, or care needs, so it is worth checking the specific requirements of the variety you plan to keep.
Northern Mountain Swordtail Habitat Setup
When setting up an aquarium for the Northern Mountain Swordtail, a high water flow best suits their natural preferences. Because this species occupies the surface and mid-level zones, the aquarium layout should prioritise adequate surface area with floating cover to make them feel secure near the waterline and open swimming space in the centre of the tank for free movement. Useful additions include dense planting to provide cover, break sightlines, and reduce stress. Keepers should note that a tight-fitting lid is essential, as this species is a known jumper.
Northern Mountain Swordtail Compatibility Notes
Classified as mostly peaceful, the Northern Mountain Swordtail is a social species that requires the company of its own kind to thrive. When choosing tankmates, their energetic swimming style may stress more sedentary or timid species. Selecting tankmates that favour different water layers from the surface and mid-level zones can help reduce competition and territorial overlap.
Northern Mountain Swordtail Recommended Tank Sizes
Standard aquarium sizes that meet the minimum volume and length requirements for Northern Mountain Swordtail.
30 Gallon Breeder
40 Gallon Breeder
55 Gallon Standard
60 Gallon Breeder
75 Gallon Standard
90 Gallon Standard
Show 10 more tank sizes
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Frequently Asked Questions About Northern Mountain Swordtail
How big does Northern Mountain Swordtail get?
Northern Mountain Swordtail (Xiphophorus nezahualcoyotl) typically reaches a maximum adult size of approximately 5.5 cm. They have a fusiform body shape and a fast growth rate.
What tank size does Northern Mountain Swordtail need?
A minimum aquarium volume of 110 litres and a tank length of at least 90 cm are recommended for Northern Mountain Swordtail. This ensures enough swimming space and helps maintain stable water conditions.
What water parameters does Northern Mountain Swordtail need?
Northern Mountain Swordtail thrives in freshwater only conditions with a temperature range of 20–25°C, a pH between 7 and 8, and a general hardness of 8–20 dGH. A high water flow is preferred.
Is Northern Mountain Swordtail peaceful or aggressive?
Northern Mountain Swordtail is classified as mostly peaceful with an aggression score of 3 out of 10. They are generally well-suited to peaceful community tanks.
Can Northern Mountain Swordtail live alone?
No, Northern Mountain Swordtail should be kept in groups of at least 3 to reduce stress and encourage natural behaviour.
Does Northern Mountain Swordtail have any special tank requirements?
Yes. Northern Mountain Swordtail benefits from plants - densely covered in the aquarium. These features help replicate their natural habitat and promote overall health and well-being.
Does Northern Mountain Swordtail need a lid on the tank?
Yes. Northern Mountain Swordtail is a known jumper, and a tight-fitting lid or cover is essential to prevent escape. Even a small gap can be enough for a determined fish to leap out.
Top Tank Mates for Northern Mountain Swordtail
These fish are strong compatibility matches for Northern Mountain Swordtail based on water preferences, temperament, adult size, and tank behavior.
Pacific Sturgeon
Acipenser transmontanus
Ideal Tank MatesMottled Loach
Paracanthocobitis botia
Ideal Tank MatesHillstream Loach
Beaufortia leveretti
Ideal Tank MatesSchwartz's Cory
Corydoras schwartzi
Ideal Tank MatesLeopard Cory
Corydoras leopardus
Ideal Tank MatesHi Fin Peppered Cory
Scleromystax macropterus
Ideal Tank MatesPlants That Suit Northern Mountain Swordtail
These plants pair well with Northern Mountain Swordtail based on shared water preferences, layout fit, and how likely they are to hold up in the same aquarium.
African Water Fern
Bolbitis heudelotii
Boivin's Aponogeton
Aponogeton boivinianus
Capuron's Aponogeton
Aponogeton capuronii
Madagascar Lace Plant
Aponogeton madagascariensis
Afzel's Anubias
Anubias afzelii
Amazon Sword
Echinodorus amazonicus
Fish Similar to Northern Mountain Swordtail
Swordtail
Xiphophorus hellerii
A popular, highly active livebearer known for the male's elongated lower tail fin (the 'sword'). They are hardy but require larger tanks than other common livebearers (like Guppies or Platies) due to their size and immense swimming energy. They require hard, alkaline water and do best with a ratio of one male to multiple females to disperse harassment.
Mountain Rainbowfish
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Dusky Millions Fish
Phalloceros caudimaculatus
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Giant Whiptail Catfish
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Neolamprologus Pulcher
Neolamprologus pulcher
An elegant but fiercely territorial African cichlid native to Lake Tanganyika. Known in the hobby as the Daffodil Cichlid, they are famous for their unique 'stepped' colony breeding behavior, where multiple generations of older siblings help raise the newest batches of fry.