A school of the peaceful (but very energetic!) Threadfin Rainbowfish will make a dazzling display in a show tank!

The Threadfin Rainbowfish Iriatherina werneri is a subtle beauty in both its color and exquisite form. The elegant beauty of these fish may be rather hard to believe when you consider the size of this fish, being only about 2 inches (5 cm) long. It’s probably even harder to believe if you have seen the stressed pet store appearance of this fish. But a well bred, healthy, adult Featherfin Rainbowfish will display many different intense colors and beautiful fins.

A school of Threadfin Rainbowfish in breeding condition is easily one of the most spectacular sights in the aquarium. The common names, Threadfin and Featherfin, reflect the elegant fins of mature males. The fins are long and flowing with lovely extended filaments on the dorsal fins.

The gorgeous coloring of the Featherfin Rainbowfish paired with a gregarious and peaceful nature makes them an excellent inhabitant for the aquarium. These fish are fairly adaptable and will live happily in a tank without aggression. They are best suited to a planted tank. This fish dislikes harsh lighting and excessive open water. They do need frequent water changes, but a school of these fish will certainly brighten up any aquarium.

The Threadfin Rainbowfish are best kept by a hobbyist with some rainbow fish experience. They are very sensitive to water condition changes. They are also slow and picky feeders because of their small throats. When they are kept in a community tank extra time needs to be taken to make sure they are getting enough food.

For Information on keeping freshwater fish, see:
Freshwater Aquarium Guide: Aquarium Setup and Care

  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Actinopterygii
  • Order: Atheriniformes
  • Family: Melanotaeniidae
  • Genus: Iriatherina
  • Species: werneri
Threadfin Rainbowfish – Quick Aquarium Care
  • Aquarist Experience Level: Intermediate
  • Size of fish – inches: 2.0 inches (5.08 cm)
  • Minimum Tank Size: 15 gal (57 L)
  • Temperament: Peaceful
  • Aquarium Hardiness: Moderately hardy
  • Temperature: 73.0 to 84.0° F (22.8 to 28.9&deg C)
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Habitat: Distribution / Background

The Threadfin Rainbowfish Iriatherina werneri was described by Meinken in 1974. They are found in Indonesia, New Guinea and Northern Australia. The Threadfin Rainbowfish have been found in rivers in New Guinea between the Merauke and the Fly River. In the Fly river they are known to travel over 500 km upstream from the mouth of the river. In Australia they are found in the swamps of the Jardine River and the Edward River. This species is not on the ICUN red list. Another common name it is known by is the Featherfin Rainbowfish.

The Threadfin Rainbowfish usually inhabits clear, slow moving waterways, grassy swamps and lagoons, all very lush in vegetation. The young fish and females will form large groups and congregate in the shoreline vegetation and fallen branches. Mature males will frequent these groups displaying their colors to the females in hopes of mating. Depending on the area these fish actually habitat determines what their fins look like. In their habitat the Threadfin will feed on unicellular planktonic algae, diatoms, insects that fall into the water, and other plant matter.

  • Scientific Name: Iriatherina werneri
  • Social Grouping: Groups – PreFemales and juveniles congregate in schools which are frequented by males.
  • IUCN Red List: NE – Not Evaluated or not listed


The Threadfin Rainbowfish is a small streamlined fish, reaching about 2 inches (5 cm) in length. The sexes have different fin shapes and color patterns. The male colors are difficult to describe because they will vary greatly based on health, diet, lighting, and even rank in the school. The basic body color is silver. The top will reflect blue and the bottom reflects orange. Mature specimens will show faint vertical stripes that run along the back bone. The first dorsal fin is fan shaped and will be some combination of red, black, yellow, red, or orange. The second dorsal fin, the pelvic fins, and anal fins are black or dark yellow. The tail is blue or clear with red tips.

Females will be a lovely honey color with greenish accents and transparent fins. Colors of young specimens and stressed specimens (like in a pet store) will be nowhere near as nice as mature, calm specimens. Also, fish who genetically originate from different rivers will have significant differences in coloration and fins.

  • Size of fish – inches: 2.0 inches (5.08 cm)
  • Lifespan: 4 years – Can have a life span of about 3 to 5 years when kept in a well maintained aquarium.

Fish Keeping Difficulty

The Threadfin Rainbowfish is actually a fairly easy fish to keep in an aquarium. These fish are also a bit easier then many of the rainbowfish to get their colors to come out. So with proper water conditions and and high quality mixes of food these fish will be a great addition to your aquarium.

Featherfin Rainbowfish are very sensitive to water condition changes. So they make for a good intermediate Rainbowfish. The hardest part of these fish is them recovering from the stress of transportation, this seems to affect them greatly. These fish are very timid and slow eaters. So if kept in a community tank they should be closely monitored to make sure they are getting enough to eat.

  • Aquarium Hardiness: Moderately hardy
  • Aquarist Experience Level: Intermediate – Very sensitive to water changes.

Foods and Feeding

Threadfin Rainbowfish are omnivores. They have tiny throats so feed on unicellular planktonic algae, diatoms, other vegetation, and small insects in their natural habitat. In captivity they should be fed a balanced diet of finely crushed flake and treated with small live foods to maintain good health. Frozen or live mosquito larvae, brine shrimp nauplii, daphnia, copepods, grindal and microworms all make good treats.

Feeding foods that are too large could result in serious injury to the fish. Buy processed food in small amounts frequently as nutrition in these products quickly deteriorates. Feed these fish several times a day and only what they can consume in under 5 minutes.

  • Diet Type: Omnivore
  • Flake Food: Yes – They have tiny throats so finely crush flake foods.
  • Tablet / Pellet: Yes – Tablet/pellet foods must be finely crush flake foods as food that is too large could result in injury.
  • Live foods (fishes, shrimps, worms): Some of Diet – Threadfin Rainbowfish will feed on mosquito larvae, brine shrimp nauplii, daphnia, copepods, grindal and microworms.
  • Vegetable Food: Some of Diet
  • Meaty Food: Some of Diet
  • Feeding Frequency: Several feedings per day – Feed two to three times daily, but give them only what they can consume in less than 5 minutes.

Aquarium Care

Although small, Threadfin Rainbowfish are very active, so need at least 15 gallons or more to swim in. The tank must be very tightly covered to prevent jumps. These fish are sensitive to their water, so the aquarist must be on top of maintenance with a constant regimen of 25-50% weekly water changes being a must. A sudden, even minor drop in pH or spike in nitrates could easily kill the delicate Threadfin Rainbowfish.

  • Water Changes: Weekly – Weekly water changes of 25 – 50%, depending on stocking density.

Aquarium Setup

The Threadfin Rainbowfish needs at least a 15 gallon tank. They require a school of at least 5 individuals to show even remotely good colors. A school of 10 or more will work best. It is wise to have at least two females per male.

As with many fish, these rainbow fish will do best in a tank that is set up mimicking their natural habitat. The substrate can be a fine gravel or sand. The tank must be densely planted with a good variety of plants. This will make them feel safe as well bring out their colors. A few plants that provide dense cover if the fish should choose to retreat include Water Wisteria, Hornwort, and Java Moss. They are small fish but are extremely active swimmers and need open space to swim unobstructed.

Their natural habitat provides a lot of direct sunlight, so placing the aquarium where they can get natural sunlight will be a plus. If this isn’t possible get a high quality light that will simulate this. These fish are great jumpers, so make sure to have a secure lid on the tank. Bogwood is also a great addition because it will releasing tannins which help get the water to the swampy and acidic conditions favored by this species.

Depending on the region your Threadfin comes from will determine if they need acidic water or alkaline water. Some algae growth on the tank will be a well enjoyed treat for this fish. For most rainbow fish there is a need to create a fairly active current, but this is not necessary with the Featherfin. They do need highly oxygenated water however, so some air stones will help with that.

  • Minimum Tank Size: 15 gal (57 L)
  • Suitable for Nano Tank: Yes
  • Substrate Type: Sand/Gravel Mix
  • Lighting Needs: Moderate – normal lighting
  • Temperature: 73.0 to 84.0° F (22.8 to 28.9&deg C)
  • Breeding Temperature: 78.0° F
  • Range ph: 5.5-7.5
  • Hardness Range: 5 – 19 dGH
  • Brackish: No
  • Water Movement: Weak
  • Water Region: Top – Threadfin Rainbowfish will spend most of their time in the upper regions of the tank.

Social Behaviors

Threadfin Rainbowfish is a peaceful fish. In a community tank these fish can be peaceful to a fault. Because of their timid gentle nature, coupled with their small size and slow feeding habits makes it hard for them to thrive with aggressive eaters. They usually do fine with other bigger rainbow fish unless you have boisterous specimens or too small a tank. Featherfin Rainbowfish are very active but not aggressive and should not be housed in a tank with fin nippers.

If the aquarium is designed appropriately and at least 5 individuals are kept, the aquarist will see a lot of this fish. They love to swim around and males love to display their intense colors and fancy fins. In a school with both sexes the males will have more intense colors. It is fine to keep multiple males in a tank because their “fighting” is mostly just a lot of fin flicking. It is advisable to keep either one male in the school, or more than three males to prevent bullying.

  • Venomous: No
  • Temperament: Peaceful
  • Compatible with:
    • Same species – conspecifics: Yes – Require a group of at least 5 individuals to show colors, but 10 or more are preferred.
    • Peaceful fish (): Safe
    • Semi-Aggressive (): Threat
    • Aggressive (): Threat
    • Safe
    • Shrimps, Crabs, Snails: Safe – not aggressive
    • Plants: Safe

Sex: Sexual differences

The males will have significantly longer fins, and also display a wider range of colors than females.

Breeding / Reproduction

Although getting the Threadfin Rainbowfish to spawn is not at all difficult in the right conditions, it is very hard to get the eggs to hatch and even harder to keep the fry alive. Soft, acidic water is mandatory for the breeding tank. The temperature should be about 78° F (26° C).

A pair of mature specimens should be selected, introduced to the breeding tank, and conditioned with appropriate live foods. The pair should be provided with either java moss or a spawning mop. The parents will continue to spawn in batches for a few days, so remove the eggs with the spawning medium as they are laid. The fry must be fed liquid fry food or microworms until crushed flake food can be eaten. See an overview of how to breed Rainbow fish in Breeding Freshwater Fish.

  • Ease of Breeding: Moderate

Fish Diseases

Most Rainbows are prone to skin flukes, parasitic infestations (protozoa, worms, etc.), ichthyobodo infection, parasitic infestations (protozoa, worms, etc.), bacterial infections (general), and bacterial disease. Goiters can be common in these fish in aquariums and normally caused from diet deficiencies, usually from not enough iodine.

Rainbowfish are extremely hardy and disease is not usually a problem in a well maintained aquarium. That being said there is no guarantee that you won’t have to deal with health problems or disease. Remember anything you add to your tank can bring disease to your tank. Not only other fish but plants, substrate, and decorations can harbor bacteria. Take great care and make sure to properly clean or quarantine anything that you add to an established tank so not to upset the balance.

A good thing about rainbow fish is that due to their resilience, an outbreak of disease can often be limited to just one or a few fishes if you deal with it at an early stage. When keeping more sensitive types of fish, it is common for all fishes to be infected even before the first warning signs can be noticed. The best way to proactively prevent disease is to give your Rainbow fish the proper environment and give them a well balanced diet. The closer to their natural habitat the less stress the fish will have, making them healthier and happy. Stressed fish are more likely to acquire disease.

For information about freshwater fish diseases and illnesses, see Aquarium Fish Diseases and Treatments. This is a great source for information on disease and treatments. It is recommended to read up on the common tank diseases. Knowing the signs and catching and treating them early makes a huge difference. Rainbow fish are very resilient.


Threadfin Rainbowfish or Featherfin Rainbowfish are available in stores and online in most places which stock other rainbowfish. This fish is usually priced a bit higher than some of the other rainbow fish species.