A robust and highly active livebearer endemic to Mexico (Goodeidae family). Unlike standard livebearers, females nourish their young via umbilical-like structures (trophotaenia) rather than yolks. They are infamous in the hobby as aggressive fin-nippers and should absolutely not be kept with slow-moving or long-finned fish (like Bettas, Angelfish, or Corydoras). They require hard, alkaline water and cooler temperatures.
Redtail Splitfin At a Glance
Redtail Splitfin Tank Requirements
Will Redtail Splitfin fit in your tank?
Care guides tell you what Redtail Splitfin needs, the next question is whether your tank can safely provide it. Test Redtail Splitfin against your tank size, maintenance needs, and the fish you already keep before you buy.
Redtail Splitfin Water Parameters
Redtail Splitfin Temperament & Compatibility
Redtail Splitfin Varieties
Each variety inherits the species defaults above. Only differences are highlighted.
Male
Males are slightly smaller but significantly more colorful, displaying a distinctive bright orange/red caudal peduncle (tail base). They possess an andropodium (not a gonopodium) for reproduction. Males are intensely active and responsible for the majority of the fin-nipping behavior.
Differs in 3 traits from base species
Also known as: Male Redtail, Red Tail Goodeid
Female
Females are notably larger, bulkier, and entirely lack the red/orange tail (usually plain silver or olive with mottled spots). They drop small batches of exceptionally large fry. They are slightly less aggressive than males but still boisterous.
Differs in 2 traits from base species
Also known as: Female Redtail
Redtail Splitfin Care Guide Summary
The Redtail Splitfin, scientifically known as Xenotoca eiseni, belongs to the Livebearers family group. Adults reach a maximum size of approximately 7 cm and have a fusiform body shape with a moderate growth rate. Classified as semi-aggressive, this species primarily occupies the surface and mid-level zones of the aquarium and produces a high level of biological waste.
To thrive, the Redtail Splitfin requires a freshwater only aquarium with a minimum volume of 110 litres and a tank length of at least 80 cm. Water temperature should be maintained between 18°C and 26°C, with a pH range of 7–8.5 and a general hardness of 10–25 dGH. A moderate water flow is preferred.
The Redtail Splitfin 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.
Redtail Splitfin Habitat Setup
When setting up an aquarium for the Redtail Splitfin, a moderate 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 an established tank with natural algae growth, as they depend on it as a primary food source and dense planting to provide cover, break sightlines, and reduce stress. Keepers should note that live planted layouts may be difficult to maintain, as this species tends to uproot or consume vegetation.
Redtail Splitfin Compatibility Notes
Classified as semi-aggressive, the Redtail Splitfin is a schooling species that feels most secure in a sizeable group, where natural shoaling behaviour reduces stress and brings out their best colours. When choosing tankmates, avoid housing with slow-moving or long-finned species that may become targets for fin nipping and 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.
Who Redtail Splitfin Is Actually For
Redtail Splitfin is best for a fishkeeper who wants a species that can settle into a calmer community setup and can provide a keeper who can commit to a proper group of at least 6. It makes the most sense for a hobbyist who will actually build around its needs, especially plants.
The most common mistake is buying Redtail Splitfin for looks first and only later discovering how its adult size, behavior, or bioload changes the whole tank plan. Another frequent problem is underestimating its social needs. A token pair or trio often behaves very differently from a real group.
Redtail Splitfin Recommended Tank Sizes
Standard aquarium sizes that meet the minimum volume and length requirements for Redtail Splitfin.
30 Gallon Breeder
40 Gallon Breeder
55 Gallon Standard
60 Gallon Breeder
75 Gallon Standard
90 Gallon Standard
Show 12 more tank sizes
Frequently Asked Questions About Redtail Splitfin
How big does Redtail Splitfin get?
Redtail Splitfin (Xenotoca eiseni) typically reaches a maximum adult size of approximately 7 cm. They have a fusiform body shape and a moderate growth rate.
What tank size does Redtail Splitfin need?
A minimum aquarium volume of 110 litres and a tank length of at least 80 cm are recommended for Redtail Splitfin. This ensures enough swimming space and helps maintain stable water conditions.
What water parameters does Redtail Splitfin need?
Redtail Splitfin thrives in freshwater only conditions with a temperature range of 18–26°C, a pH between 7 and 8.5, and a general hardness of 10–25 dGH. A moderate water flow is preferred.
Is Redtail Splitfin peaceful or aggressive?
Redtail Splitfin is classified as semi-aggressive with an aggression score of 6 out of 10. They can work in community setups but may clash with certain tankmates.
How many Redtail Splitfin should be kept together?
Redtail Splitfin is a schooling species and should be kept in groups of at least 6. Larger groups help distribute aggression and encourage natural schooling behaviour.
Does Redtail Splitfin have any special tank requirements?
Yes. Redtail Splitfin benefits from plants - densely covered and established algae (otocinclus) in the aquarium. These features help replicate their natural habitat and promote overall health and well-being.
Useful setup supplies
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Guidarium Editorial Desk
Reviewed against Guidarium care, stocking, and compatibility standards. Read the editorial policy.
- Last reviewed
- May 11, 2026
- Last updated
- May 11, 2026
- Issues or corrections?
- Contact the editorial team
Plants That Suit Redtail Splitfin
These plants pair well with Redtail Splitfin based on shared water preferences, layout fit, and how likely they are to hold up in the same aquarium.
Afzel's Anubias
Anubias afzelii
Anubias Barteri
Anubias barteri
Balansae
Cryptocoryne crispatula
Bonsai Rotala
Rotala indica
Broadleaf Crinum
Crinum natans
Broadleaf Sagittaria
Sagittaria latifolia
Fish Similar to Redtail Splitfin
Butterfly Goodeid
Ameca splendens
A robust and highly active splitfin livebearer native to the Ameca River basin in Mexico, where it is considered critically endangered or extinct in the wild. They are notorious fin-nippers and can be very boisterous, making them unsuitable for peaceful community tanks. They thrive in hard, alkaline water and require a strong vegetable component (like algae) in their diet.
Tequila Splitfin
Zoogoneticus tequila
The Tequila Splitfin is a critically endangered Goodeid livebearer endemic to a single spring system in Mexico. Famous for being saved from extinction by the aquarium hobby and successfully reintroduced to the wild. They are active, robust fish that require cooler water and alkaline conditions. They can be slightly nippy and are best kept in a species tank or with fast-moving, similarly sized tank mates.
Northern Mountain Swordtail
Xiphophorus nezahualcoyotl
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.
Sentani Rainbowfish
Chilatherina sentaniensis
A critically endangered and highly active shoaling fish endemic to Lake Sentani in Papua, Indonesia. Known for their energetic, almost chaotic swimming patterns and ravenous feeding habits. While rarely seen in the general trade, they are highly prized; males develop spectacular blood-red to pinkish hues and arched backs as they mature. Requires a large aquarium with significant open swimming space and clean, oxygen-rich water.
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.
Red Rainbowfish
Glossolepis incisus
Native to Lake Sentani in Indonesia, this species is famous for the brilliant salmon-red coloration of mature males. They are large, active schooling fish that develop a distinct high-arched back as they age. Females are vastly different in appearance (olive-gold). They require hard, alkaline water and plenty of swimming space.