The Mexican Tetra is a unique characin existing in two forms: a sighted river form and a blind cave form. Hardy and easy to keep in the home aquarium.
Overview
Astyanax mexicanus is one of the most unusual species in the aquarium hobby, known primarily for existing in two radically different forms. The surface (river) form is an ordinary silver tetra with normal eyes, found in rivers and streams of Mexico and southern Texas. The cave form is a completely blind fish with a pinkish-white, unpigmented body that evolved over millions of years in the total darkness of the limestone caves of the Sierra de El Abra.
The loss of eyes in the cave form is one of the most famous examples of regressive evolution. Fry are born with rudimentary eye buds that gradually become covered by skin as they grow. Instead of sight, the fish navigates using an extraordinarily developed lateral line system, sensing the slightest water vibrations.
Both forms grow to 9–12 cm and live 4–5 years. These are active, energetic fish that spend the day in constant motion. Despite their dramatic differences in appearance, the river and cave forms belong to the same species and can interbreed.
Tank Requirements
Keeping a school of Astyanax mexicanus requires an aquarium of at least 100 liters. These are active fish that make full use of all available space. Optimal water parameters are: temperature 20–25 °C, pH 6.5–8.0, and general hardness GH 4–20.
Note the temperature range — the Mexican Tetra prefers cool water and does not tolerate prolonged keeping above 26 °C. This is an important distinction from most tropical tetras.
For the surface form, a standard setup works well: dense vegetation along the sides, driftwood, and open swimming space in the center. For the cave form, create a darkened aquarium with rock caves, ceramic grottos, and minimal lighting — this allows the fish to feel at home in a natural-like environment. Use sand or fine gravel as substrate. A lid is essential, as these tetras can jump.
Feeding and Diet
Astyanax mexicanus is an omnivorous species with an excellent appetite. In the wild, its diet includes insects, small crustaceans, algae, and organic detritus. In the aquarium, these fish are completely unfussy and accept virtually any food.
The staple diet should consist of quality flakes or granules for tropical fish. Regularly supplement with live and frozen foods: bloodworms, brine shrimp, and daphnia. Vegetable matter is also important — spirulina flakes, blanched spinach, or zucchini.
The cave form has enhanced olfactory senses and finds food by smell just as quickly as sighted fish. Feed twice daily in small portions. When keeping the cave form with other species, ensure it gets its share of food — sighted tankmates may be faster.
Care and Maintenance
The Mexican Tetra is one of the hardiest aquarium fish with a care level of 2 out of 5. It is tolerant of a wide range of water parameters and adapts quickly to new conditions.
Weekly water changes of 20–25% will maintain stable water quality. Filtration should be standard for a tank of this size, with moderate flow. Monitor nitrate levels — they should not exceed 30 mg/l.
Mexican Tetras should be kept in a school of at least 6–8 individuals. In a small group, the fish may become nervous and nip at the fins of tankmates. In a sufficient school, this species behaves much more calmly. Keep in mind that their behavior is rated 3 out of 5 — these are active fish that can pester slow-moving neighbors.
Compatibility
Astyanax mexicanus is an active schooling fish with a moderate temperament. It coexists well with energetic species of comparable size but may nip fins of slow-moving and long-finned fish.
Good tankmates include Tiger Barbs, Black Skirt Tetras, Rosy Barbs, Zebra Danios, Corydoras, and Bristlenose Plecos. Mexican Tetras also do well with small peaceful cichlids such as Convict Cichlids, provided there is sufficient tank volume.
Avoid keeping them with slow fish that have long, flowing fins — guppies, bettas, and veil-tail angelfish. Very small species that large tetras may view as food should also be avoided.
Breeding
Breeding Astyanax mexicanus is not particularly difficult — rated 3 out of 5. This is an egg-scattering species that broadcasts eggs in the water column. For spawning, prepare a separate tank of 40–60 liters with fine-leaved plants or a spawning mop on the bottom.
Water parameters in the breeding tank: temperature 24–25 °C, pH 7.0, and moderate hardness. Feed the breeding pair generously with live foods for 1–2 weeks before spawning. Spawning usually occurs in the morning — the female scatters hundreds of small eggs among the plants.
After spawning, the adults must be removed immediately — they will readily consume their own eggs. The larvae hatch within 24–36 hours, and after another 3–4 days the fry become free-swimming. First food should be infusoria, then baby brine shrimp. The fry grow quickly and reach 2–3 cm by two months of age.
An interesting fact: when cave and surface forms are crossbred, the first-generation offspring have eyes, confirming that "blindness" is a recessive trait.