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Blind Cave Tetras – Astyanax mexicanus


Blind Cave or Mexican Tetra

Tetras are amongst the most popular aquarium fishes. There are probably more than 150 distinct species of tetra from which the aquarist may choose and this includes a large number of visually stunning fishes that are bound to enhance any home aquarium.

Is the Blind Cave Tetra (or Mexican Tetra) a good community fish? The Blind Cave Tetra should NOT be considered to be an excellent community fish because it can be quite aggressive and is best kept in a single-species tank. 

Key Facts

CategoryKey Facts
Common name(s)Blind Cave Tetra – also known as:
Mexican Tetra
Scientific nameAstyanax mexicanus
FamilyCharacidae
Originate fromMexico and north Central America
Care requiredEasy to care for and hugely popular
TemperamentKnown to be fin-nippers – single species tank is recommended.
Colour & FormPale body lacking in pigmentation; eyes become covered shortly after hatching; females tend to be larger than males.
LifespanUp to 5 years
Adult size4.75 inches
DietOmnivorous – eat aquatic insect larvae in nature
Aquarium size36 inches in length or greater
Compatible withCan be kept with other species but are known fin-nippers and can be aggressive
Avoid keeping withSingle-species tank is recommended
BreedingEasy if you put the fish in the right environment.
Water temp68 to 77 Fahrenheit
Water pH6.8 to 8.0
Water hardness (dGH or dH)Up to 30 dGH

Origins of the Blind Cave Tetra

Tetras, as a “family” of fishes belonging to the biological family Characidae are found in nature in Africa, Central America and South America.

The Blind Cave Tetra originates from caves in Mexico and also from northern Central America. The vast majority of Blind Cave Tetras available to aquarists are captive-bred. 

Characteristics of the Blind Cave Tetra

As you can see, the Blind Cave Tetra is an unusual fish which will grow to around 4.75 inches in the aquarium and live for around four to five years. 

The Blind Cave Tetra is a deep-bodied fish and takes the form of a tetragonal shape. The female tends to be larger than the male and with a fuller body and, once gravid (carrying eggs) will become even more rounded and, at that stage, is even more easily distinguished from the male.

The Blind Cave Tetra is distinguished by being its very light colour, as its body is lacking in pigmentation and, of course, the fact that its eyes are covered. Whilst the eyes of fry of the Blind Cave Tetra are obviously present after hatching, they become covered after a few weeks.

Less obvious is the fact that Blind Cave Tetras have a much greater sense of smell because it has taste buds all over its head. In addition and because, in nature, its food supply can be unreliable and erratic, the species is able to store around four times the amount of energy as fat compared with its surface-dwelling cousins.

The Blind Cave Tetra navigates by using its lateral line to provide feedback as to its surroundings. It is felt that the Blind Cave Tetra adapted from a river fish which subsequently came to inhabit caves in northern Mexico and adapted by enhancing the use of its lateral line at the expense of its eyes. It seems logical that this species would tend to be more aggressive towards what may be predators, as it cannot physically see them and acts accordingly to protect itself so, for this reason, a single-species tank is recommended.

It goes without saying that lighting the aquarium of Blind Cave Tetra is unimportant. In nature, living in caves, the Blind Cave Tetra is far less likely to encounter plants, as there is little to no light so it is better adapted to consuming insect larvae but in the aquarium, it is noted that it is omnivorous.

The Blind Cave Tetra tends to inhabit the middle of the aquarium. That said, it is a vigorous fish when breeding and will readily traverse the entire aquarium.

The Blind Cave Tetra is not, by nature, a shoaling fish but it is generally recommended to purchase six or more fish, as their nature is to swim together as a community and they will tend to thrive much better as a shoal. Blind Cave Tetras do have a reputation for being aggressive so they may squabble amongst themselves initially, apparently to establish a hierarchy.

It is generally advised that the minimum tank size for Blind Cave Tetras should be one of 36 inches in length or more due to the larger size of the species, which will enable a small shoal to move around freely. The tank does not need to be well-planted, rather it should resemble the cave environment which is the natural habitat of the species.

Blind Cave Tetras prefer fairly neutral water with a pH of 6.8 to 8.0, cooler than many tropical species (up to around 77 Fahrenheit and up to 30 dGH.

It is somewhat difficult to determine with certainty the sex of most Tetras. It is, however, easy to determine the sex of Blind Cave Tetras once a female is gravid (carrying eggs), as her belly becomes much more rounded.

The general rule for Tetras is that by keeping six or more of the same species in an aquarium they will be fully aware of which is male and which is female and they will act accordingly.

Most (but not all) Tetras have an additional fin which identifies them as being Tetras and the Blind Cave Tetra is no exception. If you look between the dorsal fin and the caudal (tail) fin of the Blind Cave Tetra you will observe a tiny, additional fin, known as the adipose fin. The purpose of this fin is not fully understood but, if it is present on a fish then you can be pretty certain that the fish is a Tetra.

Tetra comes from the Greek word “Tetragonopterus” which means square-finned and appears to relate to the four fins on vertical plane of the fish (dorsal, adipose, caudal and anal fins) which span the central line of the fish (when viewed from above or below, front or rear) and are not present as a pair (e.g. the pectoral fins).

Blind Cave Tetras – Video

How do Blind Cave Tetras breed?

Tetras, in general, will scatter eggs by laying them over fine plants such as Cabomba, Fontanalis or Java Moss.

As the female Blind Cave Tetra becomes ripe with eggs, the difference between the sexes becomes even more evident. If you plan to attempt to breed Blind Cave Tetras then it is recommended that you have a breeding tank prepared. Such a tank can be empty but you may wish to include a mesh or grid under which the fertilized eggs will fall.

I have observed Blind Cave Tetras breeding readily and vigorously in quite neutral water. 

The female will swim around the tank and, if you include two males then they will encourage her to lay her eggs by bumping into her. She will lay her eggs which will immediately be fertilized by the male(s) and will fall to the bottom of the tank (preferably through the mesh or trap). Once spawning is complete, remove the adults, as they are likely to consume the eggs, given the chance, and take no further parental responsibility.

Breeding tank for Blind Cave Tetras

You should prepare a tank of around 20 gallons in size with mature water. The water should be at a pH of around 6.8 to 8.0, a dGH of 18 and, ideally around 75 degrees Fahrenheit.

You may wish to introduce mosquito larvae or bloodworm as an inducement to reproduction.

The female will swim amongst the plants, laying her eggs whilst the male will swim alongside or behind her and fertilize the eggs as they are laid. Typically, the female can lay up to one thousand eggs, which will to the bottom of the tank.

Once the female has scattered her eggs and the male has fertilized at least some of them then the adults should be removed carefully from the breeding tank because they will have nothing more to do with the eggs but they may simply eat them.

Keep the lights off and the tank dark because the eggs and fry are particularly sensitive to the light.

The eggs will hatch typically in around 24 to 72 hours depending on tank temperature and conditions and the fry will become free-swimming after around one week after hatching. Keep the tank unlit for the first week or so then gradually increase the lighting.

You will notice that the fry have eyes at hatching but, over several weeks, their eyes will become covered and will serve no purpose

The newly hatched fry will firstly feed on their yolk sac but, once free-swimming, can be fed infusoria (particularly rotifers) and will also thrive on egg yolk during the first two to four weeks. It is worth mentioning that immediately after hatching, fry seem quite vigorous but will soon go into a resting state before they become free-swimming so please don’t mistake this initial stage as being free-swimming.

After around four days or so add baby brine shrimp. Once the fry are sufficient in size not to be treated as a snack then they can be introduced into the community tank where they will join the existing shoal. Before moving the adolescent fish into the community tank ensure that you have balanced the water temperatures to mitigate the risk of White Spot or other diseases being triggered.

Should your Blind Cave Tetras have a special diet for breeding?

Adult Blind Cave Tetras don’t need any particular inducement to breed. That said, it has been suggested that adding mosquito larva may encourage them, presumably because the addition of a new food may “fool” the fish into thinking that it is breeding time. 

Mike Wheeler

I started keeping freshwater tropical fish in 1972 and it has been something of a passion ever since. In this website, my aim is to build up an everyman's guide to help the everyday aquarist get the best from this inspiring and entertaining hobby.

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