The rainbow tetra fish (Nematobrycon lacortei) is a freshwater fish classified in the order Characiformes and Characidae family. It is believed to have been first discovered and described between the late 70s and early 80s.

Fishkeepers love rainbow tetras because of their beauty and peaceful demeanor. In this article, I’ll share my knowledge and experience on successful breeding and care for rainbow tetras with details on diet, tank set-up, natural behavior, good tank mates, and common problems.

Rainbow tetra fish.

What is a rainbow tetra?

The rainbow tetra is a freshwater characin fish of the Choco region of Colombia, South America. It is native to the Río San Juan river system, specifically, the Río Calima tributary found in the western parts of Columbia.


  • Class: Actinopterygii
  • Order: Characiformes
  • Family: Characidae
  • Genus: Nematobrycon
  • Species: Nematobrycon lacortei

Actinopterygii are ray-finned fish.[1] The generic name, Nematrobrycon, originates from ancient Greek and means thread in reference to the thread-like rays in the caudal fin.

Expert tip: Remember that the native habitat of rainbow tetras is the slow-moving river of western Columbia. Since these rivers are full of vegetation, we want to consider that when setting up the tank for the fish.

Appearance and identification features

Rainbow tetras are small, slender fish that grow to about 2 inches long. They are colorful with a rainbow-like pattern on their bodies. The body is long and cylindrical, ending in a forked tail.

The distinctive physical feature of rainbow tetras is the characteristically blue or silver body with a series of horizontal stripes along its sides. Fins are transparent, but it is common to spot a few orange or red markings.

The scales, therefore, reflect light and produce different colors of the rainbow, hence the name rainbow tetra fish. You’ll notice a dark horizontal line running along the body from the head to the tailfin.

Expert tip: Eye and body color are key to identifying sexual dimorphism in rainbow tetras. Males have vibrant red eyes, while females have bluish-green eyes. In addition, males have more vibrant coloration with brighter reds and blues, while females have a more subdued coloration with less vibrant reds and blues.

In my experience, eye color is not reliable for telling the gender of rainbow tetras. Both males and females can have a variety of colors, including red, blue, green, and black. Also, the males will only show increased iridescence at sexual maturity.

Scientific nameNematobrycon lacortei
OriginRío Calima river, Columbia
TypeFreshwater fish
Lifespan3-5 years
Average size2 inches
Temperature72°F to 80°F
Water pH6.0 to 7.0
Minimum tank size20 gallons
BehaviorSchooling fish

Care for rainbow tetras

Nematobrycon lacortei are relatively easy to care for, but I wouldn’t say they’re a good choice of aquarium fish if you’re a beginner. Their aquarium must be set up with plenty of vegetation, which means there will be plenty of decaying organic matter that can alter the water chemistry quite often.

Although the tetras are hardy, they still need proper care to remain healthy and happy.

Here are the care requirements to follow:

Tank size

Rainbow tetras can be kept in various aquarium sizes, but a 20-gallon tank is the minimum recommended size for a school of six fish. Their schooling behavior is important, so you should keep them in groups.

While you can successfully keep a few rainbow tetras in a 15-gallon tank, I wouldn’t recommend it. The minimum tank size is 20 gallons with plenty of hiding spaces, swimming room, and plants.

Water quality

Water quality is key for healthy fish. Rainbow tetras are best adapted for life in slightly acidic water. Before introducing rainbow tetras, test the water to ensure it meets the parameters within which the fish can thrive.

Here are the ideal water parameters for rainbow tetras:

  • Water pH level: 6.0 to 7.0
  • Water temperature: 72°F to 80°F
  • Water hardness: 5-8.0 dGH
  • Ammonia: 0 ppm
  • Nitrite: 0 ppm
  • Nitrate: <20 ppm

Keep the water clean and well-oxygenated and the hardness consistent. Changes in water hardness, temperature, and acidity can stress your fish and cause undesirable symptoms, including:

  • Irritability
  • Lethargy
  • Loss of appetite
  • Loss of color
  • Fin rot

It is best to keep your fish in water similar to their natural habitat.


Rainbow tetras are omnivores and will feed on most aquarium fish foods. Feed them pellets, floating flakes and supplement them with live foods such as daphnia and brine shrimp. Bloodworms and mosquito larvae are also great treats for the tetras.

Avoid overfeeding the fish in an attempt to fatten them. Simply provide food that they can consume within 2 minutes. Putting excess of it in the tank will increase ammonia levels and call for frequent cycling.

Tank setup

The natural habitat of rainbow tetras is characterized by slow-moving water and plenty of vegetation. Since they originate in freshwater streams and rivers, these tetras require careful tank set-up that replicates their natural habitat as much as possible.

The ideal situation is to create a spacious, mature, softwater planted aquarium.

Select a tank that’s 20 to 30 gallons. Despite their small size, male rainbow tetras can be particularly showy when attracting females for mating, which is why they require enough room. Smaller quarters can become problematic when the males begin to spur with each other as they compete for females.


Add soft sand to the tank as the main substrate. You can also litter the sand floor with dried Indian almond leaves. Their tannins are believed to act as an antibacterial in the tank.

Decorate the tank adequately, placing plenty of rooted and floating aquarium plants in the tank. You can also add some pieces of driftwood to improve the natural feel of the tank and make it look like their natural habitat.


If you’re placing the tank in a dark room, provide an aquarium light to keep your plants alive and maintain the circadian rhythm of your fish. I recommend a fluorescent light that puts out about 2 to 5 watts per gallon of water.

You also want to keep the lights on for at least 9 hours daily, as this is healthy for your rainbow tetras.

Expert tip: Ensure your new tank is well-cycled to avoid the new tank syndrome problem that often kills fish within a short period of being introduced into the tank.


Ideally, keep 6 rainbow tetras in a tank of 20 gallons, one male and 5 females. Males can sometimes be territorial, so having a healthy male-female ratio will reduce aggression.

Here’s a general stocking guideline:

Tank sizeNumber of rainbow tetras
20 gallons6 (1 male and 5 females)
30 gallons9 (2 males and 7 females)
40 gallons12 (2 females and 10 males)
55 gallons16 (4 males and 12 females)

Tank mates

Rainbow tetras are peaceful fish and can be kept with various other fish. Since they like to dwell and swim in the midwater region of the tank, a general rule of thumb is to keep rainbow tetras with peaceful, bottom-dwelling fish such as corydoras catfish.

You can also keep them with other species of similar size and temperament. 

Some good tank mates for rainbow tetras include:

  • Apistogramma cichlids
  • Platies
  • Mollies
  • Mikrogeophagus spp (cichlid)
  • Dwarf Loricariidae species
  • Otocinclus catfish
  • Snails

Most fish in the same family of Characidae can also count as compatible tank mates so long as they can live in the same water parameters as rainbow tetras.

Avoid keeping rainbow tetras with aggressive fish such as Oscar fish, aggressive cichlids, or barbs.


Rainbow tetras are schooling fish, meaning they swim closely together to forage for food and protect each other from predators. Keep them in groups of at least six to encourage this behavior and make them comfortable. They’re active swimmers and are often seen chasing each other around and exploring their surroundings.

Stocking the tetras in groups that can school also helps prevent stress and misdemeanors in the tank. Fewer fish will be stressed, which makes them aggressive and territorial.

I’ve seen some stressed rainbow tetras nip and bully other same-sized or smaller tank mates, so ensure you keep these fish in groups to allow for distractions and playmates.

The fish are not shy, so it is normal to find them at the front of the tank, exploring vegetation, decorations, and anything new in the aquarium.

Owing to their beautiful shimmer and style of movement, rainbow tetras make for an amazing addition to any tank, especially one that’s placed for display and decoration.


Rainbow tetras are peaceful and social species that can be kept in community tanks. They’re generally non-aggressive towards other fish and are safe to keep with other peaceful fish like corydoras and other small-sized species of tetras.

However, other factors can influence the temperament of Nematobrycon lacortei, including the size of the tank and the presence of other fish. The fish can become stressed and aggressive if kept in a small tank or if they feel threatened by their tank mates.

Overall, rainbow tetras are peaceful as long as their care needs are met, and their tank is not overcrowded.

Average size

The average size of an adult rainbow tetra is 2 inches long. Some fish can grow as big as 3 inches, while others can be as small as 1.5 inches.

Being small fish, it is best to keep them in groups to encourage their shoaling behavior. As small fish swim in the aquarium, they create a shimmering spectacle that’s quite a sight. 


The average lifespan of rainbow tetras is 3-5 years in captivity. They can live for up to 6 years or slightly longer with optimal tank conditions and proper care.

Here’s how you can help your rainbow tetras live longer:

  • Do weekly water cycles, changing 25% of the water to keep it healthy.
  • Use a filter in the tank.
  • Provide hiding places in the tank (plants, rocks, and driftwood)
  • Maintain a consistent water temperature of 72 to 80°F. This keeps their immunity in good condition.

Expert tip: Most aquarium fish die due to poor water conditions, new tank syndrome, and poor immunity. Stressors are known to compromise the immunity of fish, so ensure the tank is as comfortable as possible for your fish.

Sexing Nematobrycon lacortei

It is quite difficult to tell the gender of juvenile rainbow tetras because prominent features are not fully developed at an early age. However, upon maturity, sexing becomes easier.

Males are higher backed with longer fins, specifically an elongated dorsal fin. They’re much more colorful than females.

In addition, males have red eyes, while females have blue/green eyes. However, eye color becomes more prominent in adult fish.

Male rainbow tetra fish with blue eyes.
See the red eyes of the male rainbow tetra (Nematobrycon lacortei)

Breeding rainbow tetra fish

Rainbow tetras are known to pair up when they reach breeding maturity. I highly recommend moving breeding fish into a separate aquarium set-up specifically for breeding. The fish are easy to breed and will do so on their own.

I find spring and summer to be the best time to breed rainbow tetras, as these months are warmer, which means less effort in trying to keep the tank conducive.

Here’s how to set up a breeding tank for Nematobrycon lacortei:

  • Set up a breeding tank with a sponge filter.
  • Add plenty of vegetative aquarium plants. You can also use spawning mops (I’ve explained the importance below).
  • Maintain water parameters at 78-80 degrees Fahrenheit and a pH level of 6.0-7.0.
  • Add two paired-up rainbow tetras into the tank and feed them adequately.

Soon enough, the female rainbow tetra will become rounder, with the belly region visibly swollen. This means she’s heavy with eggs and is ready to lay.

She will lay her eggs all over the tank, and the male will fertilize them. The eggs hatch within 36 to 48 hours. Remove the adult fish and feed the fry a diet of baby brine shrimp, infusoria, or daphnia.

Note: Rainbow tetras don’t take care of their fry. I’ve seen some of them eat their eggs and even their young ones. The dense vegetation in their breeding tanks helps to increase their survival chances as they provide hiding spaces for the fry.

Common problems and diseases

Most problems with rainbow tetras originate from poor care, inhabitable water conditions, and improper tank sizing. 

Some common problems with rainbow tetras include:

  • Ich: Ich is a common parasitic infection that can cause white spots on the fish’s body. It is caused by poor water conditions and can spread from other infected fish.
  • Fin rot: Fin rot is a bacterial infection that causes the fins to rot. The main cause is poor water quality. Stress can also lead to poor immunity, putting fish at risk of fin and tail rot disease.
  • Fin nipping: While tetras are known to be peaceful little fish, they have well-developed teeth. When stressed or housed in an overcrowded tank, they’ll happily use their teeth to nip at other fish, especially those with long fins.


What is the breeding behavior of Nematobrycon lacortei?

Nematobrycon lacortei are egg scatterers and will lay their eggs on plants or other surfaces in the tank. The eggs will hatch in about 36 hours, and the fry will start swimming freely in about 3 days.

How can I prevent diseases in rainbow tetras?

You can prevent diseases in Nematobrycon lacortei by keeping the tank clean and well-maintained, using a filter to remove waste from the tank, and testing the water regularly for ammonia, nitrite, and nitrate levels. Also, ensure you quarantine new fish before adding them to a community tank to avoid spreading diseases.

How can I treat diseases in rainbow tetras?

If you suspect your rainbow tetras are sick, consult a veterinarian immediately. They will diagnose the fish and recommend a suitable treatment.



Rainbow tetra fish, sometimes called emperor fish, are beautiful, peaceful, and easy to care for. They’re native to South America and come in various colors including red, green, blue, and yellow. Since they’re schooling fish, keep them in groups of at least six.

Regarding diet, rainbow tetras are omnivorous and will eat common fish foods, including flakes, pellets, and live food.

Although hardy, these fish do well in water parameters of a pH range of 6 to 7 and a temperature range of 72 and 78 degrees.


  • [1] Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2023. The Animal Diversity Web (online). Accessed at
  • [2] Weitzman, Stanley H. and William L. Fink. “A new species of characid fish of the genus Nematobrycon from the Rio Calima of Colombia (Pisces, Characoidei, Characidae).” (1971).

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