Alfaro cultratus, also known as the knife livebearer, is a Central American live-bearing species that is quite popular among aquarium keepers with an interest in fish breeding.

They are a subtly attractive schooling species that are ideal for keepers with experience caring for fish that are sensitive to water conditions.

Keep reading to learn the best practices for the care, maintenance, and breeding of your knife livebearers.

Alfaro Cultratus Stats

Listed tank sizes are the minimum

Size: Average adult size: 3–4″ (8–10cm)
Tank: Minimum 10 gallons
Strata: Top
pH: pH recommendation 6.0 to 8.0
Hardness: Soft to medium (will adapt to a range of specifications)
Temperature: 75 to 79°F (24 to 26°C)
Order: Cyprinodontiformes
Family: Poeciliidae
Sub Family: Ancistrinae
Genera: Alfaro
Species: A. cultratus

Origin and Appearance

The knife livebearer is a species of tropical freshwater fish native to Costa Rica, Panama, and Nicaragua. Its name comes from the interlaced, knifelike pattern of its scales and its method of delivering their young. A member of the family Poeciliidae, this species gives live birth as opposed to laying eggs.

Knife livebearers are most often found in shallow, moving water over vegetation. Like most Central and South America freshwater tropicals, they are a blackwater species found in the tannin-stained waters of rivers and streams.

It is a schooling fish with a preference for clean, moving water. Their name comes from the rows of overlapping, knife-shaped scales towards the tail of the fish.

While knife livebearers lack the colorful hues of species like tetras, they are a good addition to a species or community aquarium for their silver sheen. Their bodies are light-colored, ranging from a silvery olive to gray on their back, shading to white on their bellies. Some color variations range from silvery blue or green.

The knife livebearer body shape tapers at the head and tail. They are built for strong swimming in currents. Their fins are most often translucent. Adult fish will have black markings on the edges of their tails or dorsal fins. Most fish will have a small red spot at the back of their gill plates.

Females of the species are larger than males. Alfaro cultratus size can reach up to three inches long. As with many South American small fish species, the Alfaro cultratus lifespan is three to five years.

What the species lacks in lifespan, they make up for in reproduction. The knife livebearer is a prolific breeder.

Alfaro Cultratus Care

There are two key factors for you to keep in mind for successful Alfaro cultratus care.

The first is that the species needs clean, moving water to thrive. The second is that knife livebearers are susceptible to bacterial disease, which can be caused by changes in water composition, a buildup of waste in the aquarium, or the introduction of disease from new additions to the tank.

Knife livebearers do not need a large aquarium. You can keep a school of 6 to 12 fish in a 10-gallon tank, although you will want to upsize if you are creating a community aquarium. Install a good filtration system with a strong water return. This is a species that lives on the top level of the aquarium, so a high return pipe is a good idea.

The ideal knife livebearer water temperature is 75 to 79°F (24 to 26°C). Depending on where you live in the world and the room conditions where you keep your aquarium, you may need to add a good quality water heater to your equipment. Monitor the water quality often to maintain a soft to medium water hardness and a pH of 6.0 to 8.0.

Keeping the water from getting stagnant is one of the most important aspects of knife livebearer care. Part of this is making sure you change about 30% of the aquarium water every other week. Other aspects include the types of plants and the cleaning tank mates you add to the aquarium.

Knife livebearers like swimming above vegetation. Their fry will hide among plants until they are big enough to avoid being eaten, so consider heavily planting the bottom third of the aquarium with good water cleaning species.

Vegetation such as Guppy Grass does a great job of helping keep the water clean and provide good habitat. Java moss and Amazon fern species also are good choices. Keep your plants trimmed so that the knife livebearers have swimming room at the top of the tank.

In addition to aquatic plants, you can consider pieces of driftwood, rocks, or aquarium décor both for the overall appearance of your aquarium and for the benefits of bottom-dwelling tank mates.

When water conditions are not ideal, knife livebearers can weaken and become susceptible to various naturally occurring or introduced illnesses. One of the most common illnesses is neon tetra disease. Neon tetra disease is a prevalent ailment in tetra species.

Named after the first species it was identified in, neon tetra disease is a parasitic infection that first cripples, then kills the fish. It can spread rapidly. If you see any diseased fish in your aquarium, you should remove them immediately.

The only way to prevent the introduction of neon tetra disease to an aquarium is to quarantine any new additions for up to two weeks before introducing them to the tank. Try to purchase fish in person and visually select healthy specimens from a reseller who will guarantee their quality. If you purchase aquarium fish online, be sure the reseller will guarantee their fish for at least 30 days.

Tank Mates

In general, livebearers are peaceful and make good community fish. However, male livebearers can be aggressive towards each other and are constantly trying to mate, so a ratio of two to three females per male is advised to prevent excessive harassment of the females.

Since they live at the top of the aquarium, a good tank mate strategy for you to follow would be to add species that live in the middle or bottom.

Here is a list of species that make good tank mates to knife livebearers and Amazon community aquariums:

  • Guppy: There are many varieties of guppy that make great tank mates for most Amazon community tanks. What they all have in common is that they are peaceful fish that are not too small and get along well with everything that will not eat them. Some of the more ornamental varieties will look great in your aquarium as long as none of the other tank mates are fin nippers.
  • Corydoras: Like Plecos, Corydoras are a variety of bottom-feeding catfish commonly placed in tropical freshwater aquariums. They grow up to four inches in size and are peaceful omnivores.
  • Cardinal Tetra: Cardinal Tetra makes great neighbors with many kinds of similarly gentle species. A small school of Cardinal Tetra adds beauty to any aquarium and does not create any type of significant bioload to the tank. Always a great choice. Add them in groups of 10.
  • Neon Tetra: Another great tank mate choice for apple snails. Adding them in groups of 10, same as Cardinal Tetra, per 20-gallon tanks will give you an ever-moving, shimmering display.

Any combination of these species will give you a colorful, active community tank that is friendly top to bottom.


As their name implies, knife livebearers reproduce ovoviviparous, which means they deliver their fry through live birth. In the wild, they follow the rainy season breeding cycles that drive most Amazonian wild fish reproduction.

In well-kept aquariums, they may spawn seasonally or year-round depending on how much natural sunlight they receive and the overall temperature of the water. Commercial breeders can maximize their breeding potential through deliberate changes in water quality and temperature, but your fish will breed on their own with little encouragement.

Knife livebearers will breed most readily in sole species or peaceful community aquariums where the fish are not stressed.

When your female knife livebearers are pregnant, you will see changes in their body shapes. Their abdomens will swell and take on a darker shade. Their gestation period is about 24 days.

If you are keeping knife livebearers in a community aquarium, you can maximize the number of surviving young by moving the pregnant females into a separate tank. Allowing them to birth in the community tank will subject the young to predation by tank mates.

The female will give live birth to anywhere from 10 to 30 young. The fry will be thin and very translucent. They will form small schools at the very top of the tank. Once the young are born, you can move the females back into the community tank. They have no parental roles and will sometimes eat the fry.

Newborns will feed right away on very small floating flakes. They grow quickly and can be rehomed in a month. They reach sexual maturity in about six months.

Proper Diet

Knife livebearers are primarily carnivores. In the wild, they eat insects from the surface. As such, you can feed them with a variety of live, frozen, and dry foods.

They are not fussy and will accept most anything offered. The species is happy with frozen, freeze-dried, or live food, and a mix of offerings keeps them from getting turned off by a mono food type.

Usually, they prefer meaty protein foods, some of which may need to be chopped to a size small enough to fit in their mouths.

Good meal choices include:

  • Tetra flake food
  • Tetra small pellets
  • Fruit flies
  • Brine shrimp
  • Daphnia
  • Tubifex


  • Alfaro cultratus is a live-bearing, warm water tropical fish species from Central America.
  • Controlling water conditions is key to the successful long-term care of the species.
  • Knife livebearers are a schooling species. They are happiest in schools of six or more.
  • They prefer clean, moving water over heavily vegetated tank bottoms.
  • Small, peaceful fish from the same river systems make ideal tank mates for knife livebearers.
  • Quarantine new tank mates for at least two weeks before introduction to ensure they do not carry neon tetra disease into your aquarium.
  • Ill or dead fish should be removed from the aquarium quickly.
  • Knife livebearers are easy to feed. They readily eat small insects or omnivore flakes that will float on the surface.

If you are an experienced aquarium keeper, Alfaro cultratus is a great species to add to your Amazonian community tank. You will appreciate their subtle beauty, schooling behavior, and ready breeding ability. Give them a try in your aquarium.

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