A tetra is a shape and a habit, not a species
Tetra fish is not the name of an animal. It is a label the hobby puts on a few hundred small, schooling South American and African fish, and the single most useful thing to know about all of them is the same: they are bought by the fish and kept by the school.
Aquarium Co-Op gives the vocabulary: tetras, also known as characids or characins from the Characidae family. PetMD, whose page is by Dr. Maria Zayas, DVM, gives the character: tetras are small, colorful freshwater fish known for their peaceful temperament and active swimming behavior.
The family half of that has just stopped being true. Wikipedia records that in 2024 the tetra families Acestrorhamphidae and Stevardiidae were split out of the Characidae, leaving it with just five subfamilies. It also notes that this family has undergone a large amount of systematic and taxonomic change, and that the definition was eventually restricted to just the American tetra type characins, which is how piranhas and dorados left it earlier.
So the care sheets are using a family name the taxonomists have moved most tetras out of. Nothing about keeping the fish changes, and we print it because a reader who looks up the family will find a different answer from the one every care page gives. Wikipedia also gives the size that explains the whole group: many are less than 3 cm, 1.2 in. That is smaller than every fish on our fish silo except the pea puffer and the one tetra we hold.
What this guide is
- Scope
- Tetra fish care as a group: what every source asks for regardless of species, and where the species differ enough that a group figure is useless.
- Sources
- Four, all already cited on this site, one of them written by a veterinarian.
- The argument
- One of those five publishes the stocking rule our own stocking page says appears in none of our sources. Station 03 is that argument, and the stocking page has been corrected.
Six is the floor, and three sources push it higher
This is the one figure every source on this guide states, and the one a shop is least likely to enforce.
- Aquarium Co-Op: you will need to get a school of at least six fish, of the same species, so that they feel safe and protected from potential predators. The phrase of the same species is doing work: six tetras of four kinds is not a school.
- Aquarium Co-Op on neons and cardinals: the absolute bare minimum you could have is 6, but it is best advised that you add even more to your tank within a few weeks.
- PetMD: tetras should be kept in groups of at least five to six fish, and they should be housed in groups of at least five, ideally in odd numbers.
- Our own ember tetra record: a group size of 8 to 10, which is higher than the group figure any of these sources gives for tetras generally.
The odd numbers claim is the one to look at twice. PetMD states it and offers no mechanism, and no other source on this guide repeats it. We print it because a named vet-reviewed source says it, and we are not going to convert it into a recommendation on the strength of one line.
The species that need more than six are named individually rather than as a rule. Aquarium Co-Op describes the rummy-nose tetra as prized for their tight schooling behavior and says its rosy coloration rapidly fades away when stressed, which makes the school size visible: a rummy-nose tetra tells you whether the group is big enough by changing color. No other fish on this site has an indicator like that.
An inch of fish per gallon, and who published it
Our stocking page is built on refusing one rule of thumb, and until this guide was written it said that rule appears in none of the sources we compile from. That was wrong, and this guide is where it gets corrected.
PetMD publishes it: a good rule of thumb is 1 gallon of water per inch of fish, with the worked example that five 2-inch-long tetras will need a 20-gallon tank or larger. The page is written by a veterinarian and PetMD is a source this site already cites on two records. So the honest position is not that nobody we read says it. It is that we read a source that says it and still will not use it, and the reasons are on the stocking page: it ignores floor space, it ignores the layer an animal occupies, and it treats an inch of one fish as an inch of any other.
The same page gives the figure we do use in the sentence before it: small species like neon tetras need at least 10 gallons. That is a minimum volume for a species, which is what every record on this site carries, and it does not need the rule of thumb to be useful.
What the rule does to a tetra specifically
Take the arithmetic seriously for a moment. Our ember tetra record puts the adult at 15 to 20 mm, about 0.6 to 0.8 inches, and asks for a group of 8 to 10 in a 10 gallon tank. By the inch rule, ten ember tetras are about seven inches of fish and would fit in a seven gallon tank. By the record, they need ten gallons and the reason is the group rather than the inches.
Now the other end. Aquarium Co-Op puts the Congo tetra at 3 inches and says it does best in fish tanks of 30 gallons or more. By the inch rule, six Congo tetras are eighteen inches of fish and would fit in eighteen gallons. The rule is out by twelve gallons on the one species where a source states a tank size for the school. That is the argument our stocking page makes in the abstract, made concrete by a fish.
Ten tetras, and the figures that separate them
Every figure below is from Aquarium Co-Op's own list except the ember tetra row, which is from our record. Nothing here is averaged and nothing is filled in.
| Species | Adult size | What its source says about it |
|---|---|---|
| Ember tetra | 0.8 in, 2 cm | Swims in the middle of the aquarium. Our record asks for 8 to 10 and a 10 gallon tank |
| Coral red pencilfish | 1.2 in, 3 cm | Surface dwelling. Wild caught, so more delicate, needing stable pristine water and quarantine |
| Black neon tetra | 1.5 in, 4 cm | Hardy and practically bulletproof |
| Glowlight tetra | 1.5 in, 4 cm | no further figure given |
| Lemon tetra | 1.5 in, 4 cm | Juveniles at the pet store look rather pale and colorless |
| X-ray or pristella tetra | 2 in, 5 cm | Adaptable to a broad spectrum of pH, GH and other water conditions |
| Cardinal tetra | 2 in, 5 cm | Enjoys warmer waters, and higher temperatures can increase their metabolism |
| Rummy-nose tetra | 2 in, 5 cm | Slender bodied, prized for tight schooling, and its rosy coloration rapidly fades when stressed |
| Silver tip tetra | not stated | Very interactive schooling fish with a high activity level |
| Congo tetra | 3 in, 8 cm | African. Does best in fish tanks of 30 gallons or more |
Read the size column and the group stops being one animal. The largest tetra on that list is nearly four times the smallest, and the source that names them puts one of them in a 30 gallon tank and says nothing about volume for the rest.
Aquarium Co-Op also splits the group by continent, and the split is about water rather than about looks: South American tetras are more popular because of their small size and relatively inexpensive price, but they often prefer softer water and lower pH environments, while African tetras tend to be larger and more accepting of a wide range of water parameters. That is one sentence and it undoes most group advice about tetra water, which station 06 is about. Our water and health page carries every band this site holds, and the fish silo shows where a tetra sits among them.
Neon or cardinal: the stripe, and where the fish came from
Telling them apart
Aquarium Co-Op gives one feature and it is decisive. On the neon tetra, the blue and red does not run the length of their body: neon tetras often feature a blue head and a red tail. On the cardinal, a blue and red stripe runs from the head to the tail.
It also states the size relationship in both directions and no figure at all: the cardinal tetra is larger than the neon tetra, and the neon tetra is smaller than the cardinal tetra. Neither sentence gives an inch, which is worth noting on a page whose main argument is about measuring fish in inches.
The part that decides which one survives your tank
The same source is direct about provenance. Cardinal tetras are typically wild caught, though you can find some that are captive raised nowadays, and these fish are harder to raise in an aquarium setting. Most neon tetras are also captive raised, which makes them more available, as they do not have to be sourced from the wild.
It prices the difference too: cardinals at $3 to $4 per fish against neons at $1 to $2. A shop selling a wild caught fish for three times the price of a captive bred one is selling a harder animal, and the price is the signal rather than the quality.
The one other wild caught fish on the list carries the same warning. On the coral red pencilfish: wild-caught, they tend to be more delicate and require stable, pristine water quality, and quarantine them in a separate area to prevent the spread of potential diseases. Quarantine appears once on this whole guide and it appears against a wild caught fish.
The group figures, and the one tetra we hold a record for
PetMD publishes water figures for tetras as a group: temperature 72 to 82 °F, and pH 6.8 to 7.8, depending on species. That last clause is the important one, and our own record shows why.
| Field | PetMD, tetras as a group | Our ember tetra record | Do they agree? |
|---|---|---|---|
| Temperature | 72 to 82 °F, and 75 to 80 from Aqueon | 72 to 82 °F | The two group figures overlap at 75 to 80 |
| pH | 6.8 to 7.8 | 5.5 to 7.0 | They share 6.8 to 7.0 and nothing else |
| Group | At least five to six | 8 to 10 | The record asks for more |
| Minimum tank | 10 gal for small species | 10 gal | yes |
| Lifespan | Up to 10 years, and two to four, on the same page | 2 to 4 years | Half of it agrees |
The pH row is the whole argument against group figures. A keeper who reads 6.8 to 7.8 for tetras and buys ember tetras has a fish whose own record tops out at 7.0. The two bands share two tenths of a point. Aquarium Co-Op's continental split says the same thing from the other side: South American tetras often prefer softer water and lower pH.
The lifespan row is a source disagreeing with itself. PetMD gives up to 10 years with proper care, depending on species, and, on the same page, that most tetras live two to four years in captivity though some species can live longer. We print both because both are on it, and our ember tetra record carries 2 to 4 years from its own sources.
A second source publishes the same pH and a narrower temperature. Aqueon gives 75 to 80 °F, pH 6.8 to 7.8, and the only hardness figure on this guide, an alkalinity between 3 and 8 dKH, 50 ppm to 140 ppm, which is carbonate hardness rather than the general hardness our records carry. It also gives a third tank figure: most tetras can be kept in aquariums of 10 to 20 gallons, but larger tanks are easier to take care of. And it publishes a water change routine nobody else does, 10% of the water every week or 25% every 2 weeks.
On feeding, both sources agree closely. Aqueon: most tetras are omnivorous, fed only what they can consume in under 2 minutes, once or twice a day. PetMD gives the only schedule the other sources leave out: tetras are omnivores and require a varied diet, fed once or twice daily, only what they can eat within one to two minutes. And on temperament it hedges in a way worth quoting exactly: tetras are generally peaceful, though some may nip fins. It does not say which.
Breeding: they scatter eggs and then eat them
Aqueon is the only source on this guide that covers it, and its account is four sentences long because that is what a group answer can support.
- They are egg layers, and extra effort is required to separate the adults from the eggs after spawning, as many tetras will eat their own eggs.
- The eggs go into plants. Males typically entice females into thick plant growth, where eggs are deposited and fertilized.
- They hatch in a few days. That is the only timescale anybody publishes, and no source here names a fry food.
That last gap is the one our breeding page exists to record. A fish that scatters eggs into plants and then eats them produces fry too small for anything you can buy, which is the same problem our corydoras record runs into and the reason our brine shrimp guide was written before any breeding guide.
The plants half of that sentence is the only planting advice any of our five sources gives for tetras, and it is about spawning rather than about comfort. Our own ember tetra record goes further from its own sources, carrying Aquarium Source calling a heavily planted environment essential and naming java moss, hornwort and anacharis. Aqueon adds the one decorating note anybody offers: dark substrates and decorations will help accent the bright colors of your tetras.
What this guide will not answer
Which tetras nip fins, beyond the one named
PetMD says some may nip fins and names none. Aqueon names one and gives the mechanism: serpae tetras, for example, can sometimes be nippy, especially when kept in insufficient numbers. That is the whole published answer for a group of a few hundred species, and it is the question a keeper with a betta or a guppy actually has.
A tank size for most of them
Two figures exist in our sources: 10 gallons for small species, and 30 or more for the Congo tetra. For the other eight species named on this page, nobody publishes a volume.
Water figures per species
PetMD gives one band for the whole group and qualifies it with depending on species. Aquarium Co-Op publishes no temperature or pH for any of its ten. The only per species figures on this site are the ones on a record, and the A to Z index is the list of which exist.
Neon tetra disease
It is the most searched health question about this group and not one of our five sources mentions it. We are not going to write about a disease from memory, and this guide says so rather than filling the gap.
Whether odd numbers matter
One source says to keep them in odd numbers and gives no reason. Nothing else we read repeats it. It is printed in station 02 as a quotation and not as advice.
How many tetra species there are
Wikipedia describes a family that has undergone a large amount of systematic and taxonomic change and that lost the tetra families in 2024. Counting the species in a group whose boundary moved last year is not something this guide can do honestly.
Tetra fish questions, answered from the sources
How many tetra fish should be kept together?
Six of the same species is the floor every source states. Aquarium Co-Op: a school of at least six fish of the same species, so that they feel safe and protected from potential predators, and on neons and cardinals it says six is the absolute bare minimum and to add more within a few weeks. PetMD says at least five to six. Our ember tetra record asks for 8 to 10, which is higher than any group figure here.
What size tank do tetra fish need?
Ten gallons for small species, from PetMD, and 30 gallons or more for the Congo tetra, from Aquarium Co-Op. Those are the only two tank figures our sources publish for any tetra. PetMD also offers a rule of thumb of one gallon per inch of fish, which this site does not use, and station 03 explains why with the Congo tetra as the worked example.
What water do tetra fish need?
PetMD gives 72 to 82 °F and pH 6.8 to 7.8 for the group, and adds depending on species. That qualification matters: our ember tetra record carries pH 5.5 to 7.0, so the group figure and the species figure share only 6.8 to 7.0. Aquarium Co-Op notes that South American tetras often prefer softer water and lower pH while African tetras accept a wider range.
What is the difference between a neon and a cardinal tetra?
The stripe. On a neon the blue and red does not run the length of the body, and it often has a blue head and a red tail; on a cardinal the blue and red stripe runs from head to tail. The cardinal is the larger of the two and typically wild caught, which Aquarium Co-Op says makes it harder to raise in an aquarium. Neons are mostly captive raised. Cardinals go for $3 to $4 a fish against $1 to $2 for neons.
Are tetra fish good for beginners?
PetMD says tetras are considered beginner-friendly and relatively easy to care for. Aquarium Co-Op is more specific about which ones: it calls the black neon tetra hardy and practically bulletproof, and the x-ray or pristella tetra adaptable to a broad spectrum of pH, GH and other water conditions. It also flags the exception, that wild caught fish like the coral red pencilfish tend to be more delicate and require stable, pristine water quality.
What do tetra fish eat?
PetMD is the only source here that answers: tetras are omnivores and require a varied diet, fed once or twice daily, only what they can eat within one to two minutes. Our ember tetra record carries the same shape of answer from its own sources, which is that the food has to be small enough for a fish under an inch long.
Are tetras aggressive or do they nip fins?
PetMD: tetras are generally peaceful, though some may nip fins. It does not say which species, and no other source on this guide names one either. That is the honest state of it, and it is the question a keeper with a betta most needs answered. Our betta tank mates guide collects what our sources will commit to on that.
Are tetras still in the Characidae family?
Most of them are not, as of 2024. Wikipedia records that the tetra families Acestrorhamphidae and Stevardiidae were split out of the Characidae that year, leaving it five subfamilies. Care sheets, including the ones on this guide, still say Characidae. Nothing about keeping the fish changes, but a reader who looks the family up will find a different answer from the one the hobby gives.
How long do tetra fish live?
Our one source gives two answers on the same page. PetMD: up to 10 years, with proper care, depending on species; and, elsewhere on the page, most tetras live two to four years in captivity, though some species can live longer. Our ember tetra record carries 2 to 4 years from its own sources.
Which tetra schools the tightest?
The rummy-nose, and it is the only fish on this site with a built-in indicator. Aquarium Co-Op calls it prized for their tight schooling behavior and adds that its rosy coloration rapidly fades away when stressed. A fish whose color reports its own comfort is a fish that will tell you when the school is too small.
Is an inch of fish per gallon a real rule?
It is published, including by a veterinarian, and this site still does not use it. PetMD gives it as a rule of thumb with a worked example. Our stocking page refuses it because it ignores floor space and the layer an animal occupies. Station 03 shows the failure with a real fish: by the inch rule six Congo tetras fit in eighteen gallons, and the source that names the fish asks for thirty.