Axolotl

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: sand.

Behavior

Plant interaction: plant safe.

Feeding

Carnivore. Earthworms (nightcrawlers are the standard choice) are the practical staple. Soft-bodied invertebrates such as blackworms and frozen bloodworm work well for juveniles. Some individuals take sinking carnivore pellets, others ignore them. Hand-feeding with tongs is the usual technique because the animal's eyesight is poor and it snaps at movement near its mouth. In the wild the diet is whatever fits: worms, molluscs, crustaceans, insect larvae, and small fish.

Nocturnal feeder; drop food after lights out.

Compatibility

• Not a fish. The animal is a fully aquatic salamander that never metamorphoses, kept in aquaria, sold through fish stores, and run on the same filtration, which is why its care information lives next to the fish
• Plan on a species-only tank. Anything small enough to be eaten will be eaten, and anything that nibbles will eventually go after the soft external gills, which leads to infection. Goldfish are sometimes suggested because they share the cool-water requirement, but they nip gills and can carry parasites the axolotl has no defences against
• Skip gravel. Substrate sized between sand and a smooth river stone will be swallowed during feeding and cause fatal impaction. Fine sand, large smooth stones too big to fit in the mouth, or bare bottom
• Cold-water animal. Above roughly 22 C is uncomfortable, sustained heat into the mid-20s is dangerous, and the tank cannot be set up like a tropical one. A chiller or a cool room is required in warm climates
• Famous for regenerating limbs, spinal cord, tail, heart tissue, and brain regions, and the giant genome adds to its appeal as a research animal
• Critically Endangered in the wild and restricted to a handful of canal systems near Mexico City. The animals on sale are essentially all from captive lines

Origin and habitat

A salamander, not a fish: Ambystoma mexicanum in the mole-salamander family Ambystomatidae, native to the high-altitude lake basin of central Mexico, sitting at roughly 2,240 m above sea level. The historical range took in Lakes Xochimilco and Chalco along with the connected Texcoco and Zumpango, but most of that habitat is gone; today the species hangs on at three sites (the canal systems of Xochimilco and Chalco and the smaller Chapultepec), inside a total extent of occurrence of roughly 467 square kilometres. Shaw and Nodder first described it in 1798 as Gyrinus mexicanus. The common name comes from the Nahuatl word built from atl ('water') and xolotl, the Aztec god Xolotl, the twin brother of Quetzalcoatl associated with twins, deformity, and the underworld; depending on which slice of the meaning is emphasized, the name has been rendered as 'water dog', 'water monster', or 'water twin'. The IUCN lists the species as Critically Endangered. The drivers are habitat loss as Mexico City has grown over the lake basin, pollution, and introduced tilapia and carp that eat axolotls and outcompete them. The species is the standard example of paedomorphic neoteny in textbooks: it never metamorphoses naturally, instead keeping its larval features (the frilled external gills, the tail fin, and a fully aquatic lifestyle) into adulthood and breeding in that form. Metamorphosis can be induced in the lab with thyroid hormone (T4), which produces a terrestrial salamander with much reduced limb-regeneration capacity. The animal does have functional lungs and can also exchange gas across the skin. Its more famous biological trick is regeneration: it regrows whole limbs, sections of spinal cord, the tail, heart tissue, and parts of the brain, and does so generally without scarring. The genome is enormous, around 32 billion base pairs, on the order of ten times the size of the human genome, with much of that bulk made of repetitive elements and duplicated genes. Wild colour is a mottled dark brown to grey-green flecked with gold or silver; the pet-trade morphs (leucistic, golden albino, melanoid, copper, GFP) are captive mutations, and the entire global captive population traces back through a small founder pool, including 34 animals collected at Xochimilco in 1863 and shipped to Paris, which seeded the European laboratory line. In 2025, researchers reported releasing 18 captive-bred individuals into restored or artificial wetlands near Mexico City; the released animals survived and put on weight, which supports the case for reintroduction work.

Breeding

Spawning is straightforward in captivity, and the usual trigger is a slow cool-down followed by warming back up, plus a shift in day length. Courtship is a follow-the-leader walk: the male moves ahead of the female, dropping spermatophores (sperm packets) onto the substrate as he goes and steering her over each one so she can collect it with her cloaca. Egg-laying follows roughly 12 to 72 hours later. The female deposits eggs one at a time on plants, ornaments, and the tank walls over the course of a day or two; reported clutch sizes vary widely, from around 100 to several hundred, and occasionally over a thousand from a large female. Each egg is wrapped in a clear gelatinous coat that buffers it against bumps and bacteria. Parents will eat newly hatched larvae if left in the same tank, so either move the eggs out or move the adults. Hatching takes about 14 to 17 days at typical tank temperatures; warmer water shortens it. Newly hatched larvae do not start feeding right away. Once they do, baby brine shrimp are the standard first food, with chopped blackworms and small earthworms coming in as they grow. Juveniles bite each other often, especially around the limbs, so separating them by size early on cuts losses. Sexual maturity is reached at around 12 to 18 months.

Common problems

Temperature dominates everything. Axolotls are cold-water animals from high-altitude lakes, and sustained temperatures above about 22 C cause stress and stop them eating; the longer that drags on, the more likely fungal infection and death become. In a warm climate a chiller or a cool room is not optional. Fungal infection on the skin or gills, showing up as white cotton-like patches usually around a wound, is the most common health complaint. The standard first response is to move the animal into a separate container for a short bath: either a salt bath at 2 to 3 teaspoons of non-iodized salt per litre of dechlorinated cool water for ten minutes, twice a day, or a tannin bath using Indian almond leaf or brewed black tea. Neither of these fixes the underlying cause, so the husbandry problem (heat, water quality, injury) has to be addressed at the same time. The skin is highly permeable, which means good water quality is essential and copper-containing fish medications are lethal. Loose gravel anywhere near the animal's mouth is the other classic killer: axolotls suck substrate up while feeding, and gravel sized for fish tanks impacts and kills them. Use fine sand, large smooth rocks the animal cannot swallow, or no substrate at all. Juveniles are casually cannibalistic and will bite off limbs, gills, and tails; the missing parts regrow, but the wounds are an infection route. House similar-sized animals together, feed properly, and separate any persistent biter.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size