Knife livebearer Care Guide: Diet, Tank Mates, Diseases, Breeding & More

Updated: December 17, 2022

The knife livebearer is a freshwater fish that is native to Central and South America.

This fish is not commonly kept as a pet, but it is a beautiful fish that can make a great addition to your aquarium.

If you are thinking about getting a knife livebearer, there are a few things that you need to know about their care. In this guide, we will go over everything that you need to know in order to successfully care for your knife livebearer.

Species overview

The knife livebearer (scientific name: Xiphophorus helleri) is a type of fish that’s native to Central America, specifically the countries of Guatemala and Honduras.

They prefer slow-moving waters with a lot of vegetation, such as canals, ditches, and ponds. This is something that’s common to many species of livebearer (such as the molly).

Knife livebearers are named for their long, knife-like dorsal fins. These fins can grow up to twice the length of the fish’s body and are used for both defense and courtship.

The main draw of the knife livebearer is its unique appearance. These fish are sure to stand out in any freshwater aquarium.


Knife livebearer

The most notable feature of the Knife fish is their long and narrow body shape. These fish are very tall (in comparison to their length) and very thin. This gives them a very eel-like appearance.

The body of the Knife fish is mostly a light brown color. This can vary a bit depending on the exact species and where they’re from. The brown is broken up by a few dark vertical stripes that run from the top of the fish to the bottom.

The fins on this fish are also quite unique. The dorsal fin is very tall and starts about midway down the body. It extends all the way to the end of the tail and has a very jagged appearance.

The anal fin is much shorter and starts closer to the head of the fish. It’s also not as tall as the dorsal fin.

The pectoral fins are small and thin. The caudal fin is forked and symmetrical.

The eyes on this fish are very large and protrude out from the head quite a bit.


The average lifespan for a Knife livebearer is 3 to 5 years. Once again, there are a number of factors that impact their life expectancy.

One of the biggest is whether or not they’ve reproduced. The act of reproducing can really take a toll on their bodies and shorten their lifespan.

The level of care they receive is also a big factor. If they’re in a well-maintained tank with good water quality then they’ll likely live on the higher end of the lifespan range.


The average size of a Knife livebearer is between 4 and 6 inches when fully grown. Some individuals may grow to be a bit larger or smaller depending on their level of care and genetic factors.


Tank Size

The minimum tank size for a single knife fish is 30 gallons.

If you want to keep a school of knife fish together, we recommend at least a 50 gallon tank.

And if you’re looking to keep other fish with your knives, you’ll need an even larger tank. We recommend at least a 75 gallon tank if you want to keep other fish with your knives.

Water Parameters

The knife fish is a freshwater species that is found in the rivers and streams of South America. In the wild, they tend to inhabit murky waters with little to no current.

This fish is tolerant of a wide range of water parameters. However, they prefer slightly acidic water with a pH between 6.5 and 7.5. The water should also be on the soft side with a hardness of 5 to 15 dGH.

As for temperature, the ideal range is between 72 and 82 degrees Fahrenheit.

One thing to keep in mind is that the knife fish is a nocturnal species. They are most active at night and prefer to spend their days hiding. Because of this, they do not require a lot of light. In fact, too much light can be stressful for them.

  • Water Temperature: 72 to 82 degrees Fahrenheit
  • pH Levels: 6.5 to 7.5
  • Water Hardness: 5 to 15 dGH
  • Alkalinity Levels: 4-8 dKH

What To Put In Their Tank

When it comes to setting up the inside of their tank, you have a few different options.

You can either go with a very basic setup or something a little more complex. It really depends on your personal preference and how much time you want to spend caring for these fish.

If you want to keep things simple, then a gravel substrate with some rocks and plants will do just fine. These fish don’t need anything too fancy and will do well in a wide range of environments.

If you want to put a little more effort into it, then you can add some driftwood and caves to their habitat. This will provide them with places to hide and feel safe. It can also make the inside of their tank look a little more interesting.

Just avoid anything too small or sharp. These fish are known to dig around in the substrate and can easily injure themselves on things like rocks or shells.

Common Diseases

These fish are relatively hardy and are not too susceptible to disease. However, there are a few things that you should keep an eye out for.

The most common disease that affects knife livebearers is ich. This is a parasitic infection that will present itself as white spots on the body of your fish.

If left untreated, ich can be fatal. However, it is relatively easy to treat if you catch it early.

Another disease to look out for is hole-in-the-head disease. This is caused by poor water quality and the presence of activated carbon in the tank.

It will present itself as one or two pits/holes in the head of your fish. While it is almost always curable, it will usually leave some scarring.

The best way to prevent these diseases is to maintain the quality of the water in your tank. A tank with clean and stable water conditions will always lead to healthier fish.

Behavior & Temperament

The knife livebearer is a peaceful fish that can get along with most other fish. It’s important to note that the knife livebearer is a livebearer, meaning it gives birth to live young.

Because of this, you need to be careful about what fish you put with them. If you put them with fish that are known to eat live young, the knife livebearer will not be able to reproduce.

The knife livebearer is a hardy fish that can live in a wide range of water conditions. It’s important to keep the water clean, but other than that, they’re not too picky.

One thing to note about the knife livebearer is that it is a fast swimmer. It’s not the fastest fish in the world, but it’s definitely faster than most. This can be a problem if you have slow-moving fish in your tank because the knife livebearer may chase them.

Tank Mates

The knife fish is a popular freshwater aquarium fish. It’s also known as the silver arowana, and it’s a member of the arowana family.

This tropical fish is peaceful and can be kept with other peaceful fish. It’s an active swimmer and does best in a tank with plenty of open space to swim.

Some compatible tank mates for the knife fish include:

  • Tetras
  • Rasboras
  • Danios
  • Livebearers
  • Other peaceful arowana species


The biggest obstacle you’ll face when trying to breed knife livebearers is finding a male. These fish are incredibly rare. In fact, they’re only found in a single pond in the world!

But, if you’re lucky enough to find one, the breeding process is pretty easy. The first step is to set up a breeding tank. It should hold at least 20 gallons of water.

Then, you’ll need to add a lot of hiding places. These fish are shy and need a lot of places to hide. caves, driftwood, and live plants are all good options.

Once the tank is set up, you can add the male and female knife livebearers. It’s best to only have one male for every two females.

These fish don’t need any special water conditions to breed. Just make sure the temperature is between 72 and 82 degrees Fahrenheit.

The female knife livebearer will lay her eggs in a cave or other hiding place. The male will then fertilize them.

The eggs will hatch in about two weeks. At that point, you can remove the adults and start feeding the fry live food. Baby brine shrimp are a good option.


The majority of the time, you won’t have any issues with your knife fish. They’re generally peaceful and don’t bother other fish in the tank.

However, there are a few things you need to watch out for.

First, make sure that you don’t have any fish in your tank that are small enough to fit into their mouths. They will eat them.

Second, if you have other fish in your tank that have long fins, the knife fish might try to nibble on them.

Other than that, they’re a great addition to any freshwater tank!