Threadfin rainbowfish 101: Care, Size, Tank Mates, Food & More

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The Threadfin Rainbowfish, also known as the Featherfin Rainbowfish, is an amazing addition to a planted aquarium. Like many other species of rainbowfish, this active, gorgeous fish is a peaceful and easygoing community member.

Threadfin Rainbowfishes are known for their long and flowing elegant filaments extending from dorsal and anal fins on mature males. Not only do they look amazing, but they are relatively small, only growing to be about 2 inches (5 cm) long. 

Unfortunately, these spectacular fish are not recommended for beginners due to their specialized diet and care requirements. They are also a more fragile species, so they must be kept in a mature aquarium with good water conditions.

That’s why we’ve put together this care guide to help you successfully keep Threadfin Rainbowfish in your home aquarium. In it, we’ll go over everything you need to know about their care, including diet, tank setup, tank mates, and more.

Let’s get started.

Species Profile

The Threadfin Rainbowfish was discovered in a rice paddy field in Western New Guinea and was described as Iriatherina werneri by Herman Meinkin in 1974. It is the only member of the genus Iriatherina.

In New Guinea, this fish is found in restricted areas of the Southern coast, between the Merauke and the Fly River, as well as Lakes Bosset and Kala. It also has been recorded in Indonesia and Northern Australia.

It was introduced to the European aquarium hobby in the same year it was described and became one of the most popular freshwater fishes in the aquarium trade. But only males were available in the market at that time. 

Since then, this fish has been bred in captivity to protect wild populations. Nowadays, both males and females are readily available. This rainbowfish is not the easiest species to breed, and the breeding process can be challenging. (We’ll talk about that later in this article.)

Scientific Name:Iriatherina werneri
Common Name:Threadfin Rainbowfish, Featherfin Rainbowfish
Tank Level: Top, mid-dweller
Care Level:Intermediate
Origin:New Guinea, Indonesia, Australia
Lifespan:2 – 3 years
Max Size:2 inches (5 cm)
Temperature:72-86°F (22 – 30°C)
PH:5.2 to 7.5
Water hardness:7 to 10 dGH
Tank Size:20g


The expected lifespan of Threadfin Rainbowfishes or Featherfin Rainbowfishes is 2 to 3 years. Several factors can affect their lifespan, including water quality, diet, and tank mates.


The appearance of Threadfin Rainbowfish is what makes them so special and attractive to aquarists. And it’s no wonder why – these cute critters are absolutely adorable.

They have a long and slender body with silvery sheen. The back is dark or blue (especially noticeable in males), while the sides are brownish. When mature, you’ll see a series of faint vertical stripes that run along the body.

Their first dorsal fin stands out even more against their bodies owing to its unique fan shape that combines different colors, including yellow, orange, red, and black. The second dorsal fin, the pelvic fins, and the anal fins are usually black or blue and often have exceptionally long filaments. The tail may be slightly red or translucent at its tips.

As we mentioned earlier, the most distinctive feature of Threadfin Rainbowfish is the delicate filaments (or Threads) that grow from the base of their dorsal and anal fins in mature males, primarily what gives them their name. These Threads can be up to half the length of the fish’s body and give them a very elegant look.

What’s more, is that their body color may vary depending on their location and habitat. Typically, specimens from New Guinea are somewhat darker than those from Australia.

Males Vs. Females

It’s pretty easy to tell male and female Threadfin Rainbowfishes apart. The males tend to be larger and more colorful, and they will develop longer thread-like filaments. 

Threadfin Rainbowfish Size

Out of all the rainbowfishes, this fish is one of the smallest. The maximum size of an adult threadfin rainbowfish is not larger than 2 inches (5 cm) but is more commonly around 1.2 – 1.6 inches (3-4 cm).

Care & Tank Setup

The Threadfin rainbowfish is recommended only for intermediate and advanced aquarists because they are quite delicate and need specific care. But don’t worry, we’ll go over everything you need to know to successfully keep these little guys in your aquarium.

In the wild, these fish usually inhabit clear, slow-moving streams, lowland swamps, and lagoons with abundant aquatic vegetation. They prefer to live in the vegetation-dominated margins of lily lagoons and small streams where the water depth is between 0.5 to 1.25 m and sometimes emerge in open water not far from the clumping plants.

The females and juveniles will form large schools and congregate in the shoreline vegetation while the males build their temporary territories in open water, but frequently occur in the vicinity of the female schools to display their colors to the potential mates.

When setting up a tank for Threadfin rainbowfish, you should try to replicate their natural habitat as much as possible.

Tank Size

The good news is that you don’t need a large tank for Threadfin rainbowfish. A 20-gallon (24″ x 12″ x 16″) tank is sufficient for a small group of 5 fish, but a large tank is always preferred, as the more fish you have in the group, the better chance you will see their natural colors and behavior. 

Water Conditions

Threadfin rainbowfish are quite sensitive to water quality and changes in their environment, so it’s important to maintain stable water conditions. A sudden, drastic change in pH, temperature, or nitrate spike can lead to stress and disease.

Depending on where they are from, you will determine if they need acidic or alkaline water. For example, specimens from northern Australia are used to acidic water with a pH level down to 5.5, while those from central to southern New Guinea prefer slightly alkaline water.

Waters in their natural habits are clear and include a lot of plants. So, this fish is best kept in a healthy and heavily planted tank.

The ideal water conditions for Threadfin rainbowfish are:

  • Temperature: 72-86°F (22 – 30°C)
  • pH: 5.2-7.5
  • Hardness: 7-10 dGH

Since they hail from slow-moving waters, Threadfin rainbowfish do not like a strong current. A light filtration system is sufficient. To help keep the water clean, it’s smart to invest in a good surface skimmer. Also, only add this fish to a mature aquarium as they are susceptible to swings in water parameters.

Make sure your tank is covered with a tight-fitting lid, as these little fish are also known to be jumpers.

Aquarium Decorations

When setting up your fish tank, Threadfin rainbowfishes like a heavily planted aquarium with floating plants and long, leafy plants that provide plenty of hiding spots, if you plan to raise fry alongside an adult group, the aquatic moss with a longer, softy, tight branch (such as a Taxiphylum sp.) will give the fry a place to hide. Well, a man-made spawning mop work amazingly. 

Floating plants such as duckweed and hornwort are appreciated, not only do they make the tank more comfortable, but also they discourage jumping.

Regarding the substrate, these fish don’t seem to be too fussy. A sandy or gravel substrate is fine. You can also add some driftwood and rocks to create more hiding spots and give the tank a more natural look, but make sure there is enough open space to let them swim unobstructed.

Don’t let the floating plants completely cover the surface of the water because they need some direct sunlight to bring out their colors. 

Food & Diet

Threadfin Rainbowfishes are omnivores by nature. In the wild, they mostly feed on diatoms and micro-crustaceans, insects floating on the water’s surface, and other plant matter.

In captivity, they will happily accept a balanced diet of crushed flake foods, which can serve as their diet’s base. To keep them healthy and bring out their colors, it’s a good idea to supplement their diet with live or frozen foods such as daphnia, microworms, copepods, mosquito larvae, brine shrimp nauplii, and grindal.

All food must be appropriately sized for their narrow throats, though they have large mouths. Large foods will cause serious health problems. Observe them when you start feeding to see how much they can eat in one sitting. A good rule of thumb is only to feed them what they can consume in 3- 5 minutes.

Threadfin Rainbowfish Tank Mates

The best tank mate for Threadfin Rainbowfish is more Threadfin Rainbowfishes. This fish is shoaling by nature and should be kept in a group of at least 6 individuals. Well, the number is barely the minimum, the more the better.

In addition, I would suggest keeping either one male in the school with several females or more than three males in a large group since two males may be “fighting” – more like a lot of fin flicking.

If they are given sufficient space and a comfortable environment to exhibit their frenetic dances and flap their elegant fins, Threadfin Rainbowfish will perform a mesmerizing and enviable display of their beauty that you’ll be glad to see every day. 

Besides their own species, Threadfin Rainbowfish can also be kept with other small and inoffensive fishes or aquatic creatures. Of course, fin nippers should be avoided at all costs.

Here are some potential tank mates of Threadfin rainbowfish:

  • Dwarf rainbowfish
  • Kubotai Rasbora (Microdevario kubotai)
  • Glowlight Rasbora (Trigonostigma hengeli)
  • Dracula fish (Danionella dracula)
  • Eight-Banded Barb (Eirmotus octozona)
  • Pygmy Cory (Corydoras pygmaeus)
  • Otocinclus Catfish (Otocinclus spp.)
  • Freshwater Shrimps
  • Freshwater Snails

Threadfin Rainbowfish Breeding

Breeding Threadfin Rainbowfishes in captivity is no easy task. Getting these fish to spawn is not at all difficult, but helping eggs hatch and keeping the fry alive can be quite challenging.

Unlike most Cichlids that are incredibly good parents, Threadfin Rainbowfish are not that great when it comes to looking after their offspring. Instead, the parents will most likely eat the eggs or fry when given the chance.

That being said, there are a few things you can do to increase your chances of success. First of all, you must determine which method of spawning you want to use.

A Species-only Tank

This is the most common way to breed Threadfin Rainbowfish for the most aquarist. In a species-only tank, a self-sustaining population can be maintained without removing the parents. 

However, this method just mimics their natural breeding environment and does not necessarily increase your chances of success. 

Don’t forget to add aquatic mosses at the top level of the water column to allow the fry to hide since they spend most of their time close to the water surface.

Set up a Spawning Tank

If you want to increase your chances of success, then you should set up a separate breeding tank. This tank should be well-planted with spawning mops or aquarium moss to simulate the role of plants in the wild – catching the eggs and providing protection from the parents. You need to check the media daily and remove any eggs you find into the incubator.

Author note: To make the spawning mop or aquarium moss float on the water’s surface, you can tie them to a piece of cork. 

You’ll need to provide them with an optimal environment and diet to successfully breed Threadfin Rainbowfish. Soft, acidic water with gentle water movement is ideal in the breeding tank. A reliable air sponge filter will work. 

In the wild, spawning usually takes place during the warmer months of the year. To encourage spawn in captivity, you should raise the water temperature towards the upper end of the comfort range suggested above.

Since a male can mate with several females, keeping one male and two females combination in the breeding tank is advised. The spawning usually continues throughout daylight hours during spawning seasons. 

The parents will not take care of the eggs or fry, and will continue to spawn, so it’s important to remove the eggs as they are laid. The fry are tiny and should be offered liquid fry food or infusoria as a first food until they are large enough to eat crushed flake food.

In summary

The Threadfin Rainbowfish or Featherfin Rainbowfish is one of the most unique and beautiful freshwater fish species available to the home aquarist. They are the perfect centerpiece fish for planted aquariums. 

These fish are available in stores and online, but they can be a little more pricey than many other rainbowfishes.

Overall, this fish may be a bit challenging for beginners, but it is definitely worth it to have these stunning creatures in your aquarium. 

As always, any suggestions or feedback is greatly appreciated. Thanks for reading, and good luck with your Threadfin Rainbowfish!


  1. Allen, Gerald R., Stephen Hamar Midgley, and Mark Allen. Field guide to the freshwater fishes of Australia. Western Australian Museum, 2002.
  2. Allen, Gerald R. “Field guide to the freshwater fishes of New Guinea.” (1991).
  3. Hansen B. (1987) Werneri from the wild. Fishes of Sahul 4(2): 165-168.
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Jeff Colt

Jeff Colt

Hello, I'm Jeff- an aquarium enthusiast with over 25 years of experience caring for a wide array of tropical fish, including koi, goldfish bettas, cichlids and more! For me: Aquariums are like jello - there's always room for more!

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