San Marcos Redtail Goodeid (Xenotoca doadrio) Pair

Common Name

San Marcos Redtail Splitfin

Origin and Habitat

Endemic to springs, ponds, small reservoirs, and seasonal streams in the Etzatlán region of Jalisco, Mexico, with a focus on the endorheic Laguna de Magdalena basin and nearby upper Río Ameca subdrainages. Sites range from clear spring pools with aquatic plants to shallow turbid ponds over mud and gravel. Several historical localities have been lost to groundwater extraction, spring diversion, and pollution. Remaining populations are fragmented.

Temperature and Water Conditions

Aim for 20 to 24 Celsius, which is 68 to 75 Fahrenheit, with strong aeration and steady flow. Wild waters in the region are generally neutral to slightly alkaline with pH around 7 to 8. We do not recommend chasing or altering water parameters except in special breeding situations. Stable conditions are far more important. Keep ammonia and nitrite at zero, nitrate low, and temperature steady. Prolonged periods above 25 Celsius, which is 77 Fahrenheit, can stress goodeids, so provide seasonal cooling when possible.

Appearance and Size

Males display an orange to red caudal peduncle with bluish iridescence on the anterior flanks and a dark shoulder blotch near the pectoral fin. Females are brown to olive with mottling. Adult size is about 3.7 centimeters standard length for males and about 4.7 centimeters standard length for females, which is roughly 1.5 to 1.9 inches. Total length is a bit greater under aquarium care.

Diet in the Wild

Wild populations graze algae and aufwuchs and pick at small invertebrates such as insect larvae and tiny crustaceans. This mixed herbivory and micro predation reflects productive shallow spring and pond habitats.

Feeding in Captivity

Offer a rotation of high quality spirulina or other vegetable based flakes and pellets, plus frozen or live foods such as brine shrimp, daphnia, and finely chopped bloodworms. Feed small portions two to three times daily. Regular greens support gut health and color, while occasional protein rich feeds condition females before brooding.

Behavior and Tank Setup

Active midwater fish that do well in groups. Keep at least six to eight with multiple females per male. A 20 gallon long tank, about 75 liters, or larger works well for a colony. Provide moderate current, high oxygen, and frequent partial water changes. Aquascapes can range from open hard bottom setups to planted tanks with hardy species such as Vallisneria and Ceratophyllum. Use secure lids because excited fish can jump during feeding or courtship. Choose peaceful neighbors from cooler mineral rich water conditions and avoid large predators or very aggressive species.

Breeding

A viviparous goodeid with internal fertilization and maternal nutrient transfer to embryos via trophotaenia, which are hindgut derived structures that function like a pseudoplacenta. Gestation is about five to eight weeks depending on temperature, and broods are modest in size. Provide dense cover with fine leaved plants for newborns and add rounded stones and leaf litter to create micro habitats. Separate late term females when possible and feed newborns finely crushed vegetable based foods and newly hatched brine shrimp.

Quick Facts

• Scientific name: Xenotoca doadrioi
• Common name: San Marcos Redtail Splitfin
• Origin: Etzatlán region of Jalisco Mexico in the Laguna de Magdalena basin and nearby Río Ameca subdrainages
• Adult size: males about 3.7 cm SL which is 1.5 in, females about 4.7 cm SL which is 1.9 in
• Temperature: 20 to 24 C which is 68 to 75 F
• pH: neutral to slightly alkaline in the wild, keep stable rather than exact
• Hardness: moderate to hard mineral rich water is preferred
• Temperament: active community livebearer that does best in groups with more females than males
• Diet: omnivore that grazes algae and takes small invertebrates
• Breeding: viviparous goodeid, provide plant cover and very stable cool conditions

Tropical Fish Co. Notes

A vividly marked goodeid livebearer from west central Mexico. Males show a bright orange to red tail base with iridescent blue on the body, while females are more subdued and patterned for camouflage. This species is a flagship goodeid for conservation minded hobbyists. It rewards steady cool water, strong oxygenation, and regular greens in the diet with confident behavior and constant activity. Keep a group in a planted tank with open swimming lanes and a brisk filter return. It is an excellent candidate for responsible colony breeding projects with careful record keeping of locality lines and exchanges through goodeid focused clubs.

Conservation Status

Assessed as Critically Endangered due to severe range contraction, habitat loss from water diversion and spring modification, organic pollution, and displacement by non native livebearers such as Pseudoxiphophorus bimaculatus. Only a handful of wild subpopulations remain. Captive conservation through managed hobby colonies and institutional programs is strongly encouraged.