Threadfin Rainbowfish – Detailed Guide: Care, Diet, and Breeding

Threadfin Rainbowfish (Iriatherina werneri) – Detailed Guide Care, Diet, and Breeding

Unarguably one of the most spectacular-looking freshwater fish species, Iriatherina werneri, commonly known as Threadfin Rainbowfish is in great demand by aquaculture enthusiasts because of its unique shape, bright coloration, as well as interesting movements.

Like most other rainbow species, Iriatherina werneri can easily adapt to most water parameters. It makes them easy to care for even for beginners. In addition, they are one of the most peaceful fish species you can ever find. 

This profile guide will give a special look into this remarkable species. You will know more about Threadfin Rainbowfish including their behavior, feeding preferences, ideal tank requirements, breeding requirements, and how to take good care of them.

Quick Notes about Threadfin Rainbowfish

Name Threadfin Rainbowfish
Other Names Featherfin Rainbowfish or Forktail Rainbowfish
Scientific Name Iriatherina werneri
Water type Freshwater
Tank size (minimum) 15 gallons (~60 liters)
Keeping Easy-Moderate
Breeding Moderate
Size 1.2 – 1.6 inches (3 – 4 cm)
Optimal Temperature 75 – 82 °F (24 – 28 °С)
Optimal PH 6.0 – 7.0 
Optimal GH 7 – 19 
Dwellers Top-Middle
Nitrate Less than 60
Diet Omnivore
Temperament Peaceful
Life span up to 4 years
Color Form Opaque, yellow, light brown, olive, or drab greenish

History of Threadfin Rainbowfish

In 1973, two German aquarists (Erich Frech and Arthur Werner) found some miniature freshwater fish in a rice paddy field in the suburbs of the small town of Merauke in New Guinea.

They transported these fish to Europe where German ichthyologist Hermann Meinken (1896 – 1976) came to the conclusion that these were an undescribed species.

In 1974, he described this species for his newly-erected genus in the German aquarium magazine ‘Das Aquarium’ and named it Iriatherina werneri in honor of Arthur Werner.    

In 1978, Dr. G. Allen discovered and described a similar fish in the Jardine River in the far north of the Cape York Peninsula, Australia. He named the found species Iriatherina jardinensis. However, it turned out that it was the same species.

Taxonomy of Threadfin Rainbowfish

Currently, the family Melanotaeniidae, commonly known as rainbowfishes, consists of seven genera, of which four are currently recognized as being monotypic.

Iriatherina werneri is the sole species of the genus Iriatherina.

Etymology of Threadfin Rainbowfish

The genus name Iriatherina is derived from the Greek words ‘Iris’ (-idos ), meaning ‘Iridiscent’, and ‘Atherinē’ the Greek name for the ‘Smelt’.

Distribution of Threadfin Rainbowfish

Threadfin Rainbowfish (Iriatherina werneri) – Detailed Guide Care, Diet, and Breeding - DescriptionIriatherina werneri is native to tropical Australia and Papua New Guinea.

In New Guinea, this species can be found between the Merauke and the Fly Rivers (more than 500 km upstream from its mouth), from lakes Bosset and Kala.

In Australia, Threadfin Rainbowfish occur in the Jardine River swamps, and in some river systems on the western and eastern sides of Cape York Peninsula. 

Habitat of Threadfin Rainbowfish

Threadfin Rainbowfish prefer the large, sluggish tributaries, heavily vegetated lagoons, dendritic lakes, and slow-flowing rivers.

These fish prefer shallow water depths between 2 – 5 ft (or 0.5 – 1.5 m) with lots of aquatic vegetation.

Description of Threadfin Rainbowfish

Threadfin Rainbowfish are a small freshwater species. The average size of the adult fish is only about 1.2 – 1.6 inches (or 3 – 4 cm) from the tip of the snout to the origin of the caudal fin. There are some reports that old males grew up to 2.4 inches (6 cm) long. However, I have never seen any of them bigger than 2 inches (5 cm) long.

Distinguishing characteristics of Threadfin Rainbowfish:

  • Their upper jaw is much shorter than their lower jaw.
  • The upper jaw does not protrude, it is rotated in place so that it opens upward.
  • The skin of the upper lip is directly united to that of the snout.
  • Each side of the upper jaw contains a single row of 5-6 small conical teeth anteriorly continuous with about 8-10 slightly larger conical teeth posteriorly extending nearly to the end of premaxillary.
  • The lower jaw contains 2 or 3 irregular rows of small conical teeth, with a few enlarged conical teeth in the outer row near symphysis; posterior three-fourths of the lower jaw are
  • Color pattern includes several types of pigment cells: opaque, yellow, light brown, olive, or drab greenish, without silvery midlateral band. Males have 6-8 dusky vertical bars on their flanks.
  • Innermost ray of the pelvic fin is connected for its entire length to the abdomen by a membrane.
  • Males have greatly enlarged and elongated first dorsal, second dorsal, anal, and pelvic fins. They are commonly more than twice their total body length. The first dorsal fin is oval.
  • In males, fins are often dark brown or darkly pigmented. The tip of the caudal fin is colored orange-red. On the contrary, the fins of females are relatively colorless compared to males.
Depending on the location they are from, Threadfin Rainbowfish may also have small differences in fins’ shape and coloration. For example:

  • Fish found in New Guinea are generally darker than the Australian variety.
  • In males, the first dorsal fin can vary in size from narrow to broad.
  • It was reported that a yellow-finned variety of Iriatherina werneri has been found in the Embley River in northern Queensland.

Lifespan of Threadfin Rainbowfish

Currently, there is no data available on the maximum lifespan for Iriatherina werneri in the wild.

However, in captivity, they can live up to 3 – 4 years, if appropriately cared for.

Their lifespan will greatly depend on the conditions they are kept in, how well you feed them, and how stressful your aquarium environment is for them. Threadfin Rainbowfish will live most in an aquarium containing low food competition and the absence of various larger or aggressive animals.

Typical Behavior of Threadfin Rainbowfish

Understanding their behavior is one of the most important aspects of caring for Threadfin Rainbowfish. It will help you create an optimal environment for them.


Threadfin Rainbowfish are very peaceful fish. They are absolutely harmless and will not bother anybody in the community tank. They do not nip at all.

If anything, they are more likely to be on the receiving end of aggression as their small size and large fins put a target on their backs from other fishes.

The males often compete for dominance but they do not make any contact. It is more like a dance where they rapidly move and flip their first dorsal fin up and down.  Once the dancing battle is won they do not chase and/or do not harass the loser.


Threadfin Rainbowfish is a social species. Although they like to be in a group, they do not stay in close proximity all the time. Occasionally, you will see them hanging out with each other. Thus, it is not schooling but rather a shoaling fish.

In the aquarium, you need at least 6 (bare minimum) of them to start. Large groups make them happier.


Threadfin Rainbowfish are very shy and timid fish. They are easily scared. The good thing though is that they also easily recover from the stress.

They are also very playful fish when they are happy. Despite their long fins, they are moving almost all the time and moving very quickly. Threadfin Rainbowfish fish are truly exciting to watch.

Warning: Threadfin rainbowfish are known jumpers. It is important to cover the tank.

Iriatherina werneri is a diurnal species. These fish rely significantly on their vision to detect predators and find food or conspecifics. However, they readily hide in darker areas of the tank in response to danger.

Placement in Tank:

Threadfin Rainbowfish will cruise through the tank all the time, however, they do stay at the top and middle parts of the tank most often.


  • Social: Yes
  • Activity: High
  • Placement: Top and middle dwellers
  • Peaceful: Yes
  • Nippers: No
  • Jumpers: Yes

Diet of Threadfin Rainbowfish

Threadfin Rainbowfish (Iriatherina werneri) – Detailed Guide Care, Diet, and Breeding - brine shrimp
photo by Peter Maguire

Because of the jaw structure, Iriatherina werneri is a surface-feeder, as the mouth is upwardly directed. They are omnivores; hence they consume both plant and animal material. 

According to the study, wild-caught Iriatherina werneri had fed on large quantities of unicellular, planktonic algae and some diatoms, but crustacea were absent. Even some mosquito larvae were too large for them.

Because of their small mouths and thin throats, Threadfin Rainbowfish only eat tiny particles of food. In the aquarium, we can feed them:

Threadfin Rainbowfish (Iriatherina werneri) – Detailed Guide Care, Diet, and Breeding - feedingNonetheless, this species does not require only live foods. They also accept frozen and commercial food as well, such as (links to Amazon):

Threadfin Rainbowfish will have no problem taking any small live or frozen food as far as it is small enough to fit into its mouth for mastication and digestion. Therefore, for example, large worms (grindal worms, bloodworms, blackworms, etc.) should be chopped into small pieces and flake food should be crumbled.

A varied diet is always best. It has been noticed that Threadfin Rainbowfish may even lose color because of that.

How to Feed Threadfin Rainbowfish?

It can be difficult to keep them fed if you have other fish species in the tank.

Threadfin Rainbowfish are pushovers and very shy. In addition, they are pretty slow at it. Thus, they can and will be outcompeted for food by most fish species.

  • They are diurnal animals, thus these fish should be fed during the day.
  • As surface feeders, their mouths are developed to catch prey above them. They rarely pick anything on the substrate. Ideally, you need some food that will stay in the water column for some time.
  • Be prepared to cut in pieces or grind up some flakes.
  • Make sure it is mostly a protein-based diet.
  • Clean up leftover food to avoid fouling the water.


  • Diet Type: Omnivore
  • Food Preference: Meat
  • Feeding Frequency: Daily

How Often to Feed Threadfin Rainbowfish?

Adults can be fed once a day whereas juveniles should be fed at least twice per day. 

Are Threadfin Rainbowfish Plants Safe?

Yes, Threadfin Rainbowfish are plant safe. They do not eat live plants.

Even more, planted tanks are highly recommended for them, especially for breeding purposes.

Keeping and Housing Threadfin Rainbowfish

Threadfin Rainbowfish require clean water. They do not tolerate well high levels of ammonianitrites, and nitrates. Therefore, before introducing them, your tank should be cycled and fully established.

Tank Size:

The small size of this fish should not confuse you. Threadfin Rainbowfish are not good for nano tanks.

Technically, it is possible to keep 5 – 6 Threadfin Rainbowfish even in a 10-gallon tank (40 liters). However, you will not see their true behavior and expression in such tanks. This tank will be too small for them.

Another problem with small tanks is that it can be really hard to keep stable water parameters even for experienced aquarists. Smaller aquaria should be used only for breeding purposes.

Therefore, a group of 6 – 8 Threadfin Rainbowfish need at least a 15-gallon (60 liters) long or even larger to show their true personality. 

If there is a choice, long tanks are better than tall tanks.

Important: I need to repeat it once again. You need to get a lid for the tank, Threadfin Rainbowfish are jumpers.

Related article:

Water Parameters:

Important:  If you are buying captive-bred (for several generations) Threadfin Rainbowfish you need to find out the water parameters they were bred in. If you can’t find this information, you will have to take the risk and presume that the breeder kept them in water parameters that are close to the natural ones.

Related article:

Temperature: These fish can survive in a wide range of temperature conditions 64 – 89°F (18 – 32°C). However, at low temperatures, they become slow, lethargic, and barely eat. In the aquarium, Threadfin Rainbowfish will thrive in warm temperature conditions of 75 – 82 °F (24 – 28 °С).

Important: It does not mean that the temperature in your tank can fluctuate that much in short periods of time! The range of their tolerance is strongly influenced by acclimation temperature. In other words, fish that are acclimated at lower temperatures can extend their lower temperature tolerance further compared to fish acclimated to higher temperatures.

pH: According to the Australia New Guinea Fishes Association, Threadfin Rainbowfish can tolerate pH range between 5.2 – 7.6. At the same time, when Dr.G.Allen described the habitat conditions of Iriatherina werneri (calm backwaters of the Jardine River rich in vegetation, mainly lilies), he also mentioned very low pH values of 5.2 – 5.8. Threadfin Rainbowfish prefer slightly acidic water.

In aquariums, the ideal pH range should be between 6.0 – 7.0.

Hardness: Hardness is very important for breeding purposes. Although this species has some preference for soft water, adult Threadfin Rainbowfish can easily adapt to hard water. This cannot be said of hatchlings, the survival rate in soft water is extremely low.

In aquariums, the ideal GH range should be from 7 to 19.


As for lighting, Threadfin Rainbowfish will prefer subdued lighting.

However, if you decide to keep these fish in planted tanks, lighting should be adapted to the needs of plants.

Related article:

Water Flow:

In the natural ecosystem, Threadfin Rainbowfish inhabit shallow waters where the current is absent or very slow-moving.

Still or very slow water currents are recommended for tanks housing this species.


No special requirements.

Generally, the substrate should be adapted to the needs of plants in your tank. Also, the substrate can play a crucial role when you need to keep your pH less than 7.0 (neutral).

For example, active substrates (such as ADA Amazonia aqua soil, Fluval Plant and Shrimp StratumAkadama-Bonsai soil, etc.) will lower the water parameter (pH).

Plants and Decorations:

The natural habitat of Threadfin Rainbowfish is characterized by the presence of lots of aquatic plants and leaf litter, and this should be replicated in an aquarium.

Plants provide good hiding spots for your fish.

You can use floating plantsfake plantsdriftwood, and rocks to provide great hiding spots for them.


Before putting Threadfin Rainbowfish into the tank, you need to at least temperature acclimate them.

Give them time to acclimatize before adding to the tank. Do not rush the process! Sudden changes in habitat can harm them.

Related article:

Breeding Threadfin Rainbowfish

Threadfin Rainbowfish are not that difficult to breed as long as conditions are good. Remember, there are 3 main factors for the successful breeding of this species:

  1. stable water conditions (high temperature and hard water),
  2. proper diet (live food mostly),
  3. rearing tank.

Interesting facts: In Iriatherina werneri, temperature affects the sex ratio (under high temperatures there are more males). In addition, due to the high demand for male rainbow fish on the pet market, it is possible to artificially to masculinize fish through embryo immersion in purwoceng plant extract.


Threadfin Rainbowfish (Iriatherina werneri) – Detailed Guide Care, Diet, and Breeding - sexingIriatherina werneri has sexual dimorphism.

  • Color. Male fish have a more beautiful and intense color than females.
  • Fins. Males have significantly larger dorsal and anal fins.


Threadfin Rainbowfish become mature when they are measuring around 1 inch (2.5 cm) long with an age ranging from 50 – 60 days.

It is recommended to have them at one male and three females ratio. Spawning usually occurs in the morning until late afternoon.

Male Threadfin rainbowfish start flaring their extravagant fins during both courtship and intrasexual displays. Their first and second dorsal fins, the pelvic and anal fins will open and close very quickly. All these movements are to attract the attention of a female.

Females clearly show a preference for males with longer fins, rather than a preference for particular males. According to the study, females spent more time observing males with longer fins.

He will do this dance near the chosen spot of the substrate and repeatedly approach the female inviting her to join him.

Note: Some aquarists use moss as a breeding ground. It also makes the process of moving eggs to the rearing tank way easier.

During spawning the female and male swim forward while wagging their tails, so that the eggs will stick to the substrate. Fertilization starts when the male spread out and shrinks up the fins.

Threadfin Rainbowfish are capable of spawning every day for 30 days. So, it is very unique when compared to other cultured fish species.

The potential number of eggs and larvae produced can be increased by 4 times and 14 times respectively when the parent is given natural food. Food should be given at satiation with a frequency of 3 times a day.


Threadfin Rainbowfish are egg scatterers, they are not livebearers.

On average, females usually lay 7±6 eggs each day. Due to the fact that they can do it for 30 days, females may produce up to 200 eggs per month.

Their eggs are adhesive and easily attach to substrate or plants.

Eggs of Iriatherina werneri are telolecithal, clear to clear golden in color. The egg size ranges from 0.73 to 1.08 mm in diameter. 

According to the study, incubation lasts 5 – 6 days at a temperature of 75 – 86°F (24 – 30°C).

  • After 24 hours. Head and tail can be distinguished, eye sockets begin to form and then the formation of somites is followed.
  • After 76 hours. The eyes are completely black, the pectoral fins are present and the embryonic movements are more active.
  • After 124 hours. Hatch. Larvae.

Unfortunately, the survival rate is barely 40%.

Results of the experiments showed that temperature affected the sex ratio of Threadfin rainbow fish.

  • 73.3 % of males at the temperature 86 – 89°F (30 – 32°C).
  • 56.7 % of males at the temperature 82 – 86°F (28 – 30°C).
  • 30 % of males at room temperature.


At a temperature of 89°F (32°C) and higher, the fry cannot survive. They should be fed at satiation at least twice a day in the morning and evening.

Although egg yolks begin to run out at the age of 3 days, it is still recommended to start feeding them right away.

Age (days) Development Size (mm) Recommended food
1-14 The body is transparent and still has the yolk. The caudal fin is still fused with the dorsal and anal fins.


3.3 – 8.5 Infusoria and Rotifera
15-25 The dorsal and anal fins are clearly visible, the fin margins are black and the caudal fin is yellowish


9 – 12 Rotifera and Artemia
26-60 The development of fins and body is complete. The fry becomes juvenile at 35 days after hatching.


19 – 21 Artemia

Note: After hatching, the newly hatched larvae start swimming vertically (jerky motion) to the surface of the water, then falling to the bottom of the container. They do that to inflate their swim bladders by swallowing air at the water surface.

Important: Larvae and fry of Threadfin Rainbowfish are very sensitive to soft water. It was noticed that hard water (15-17 GH) improved the survival rate by a huge margin.

Rules of Rearing tank for Threadfin Rainbowfish

  • Move eggs to the rearing tank to avoid potential
  • Water parameters: ph 7.0 – 7.5, GH 15 – 17, temperature 75 – 82 °F (24 – 28 °С).
    Note: As I have already mentioned earlier, hardness is extremely important for fry survival.
  • Once they are big enough to eat Artemia, you can start lowering the hardness. In a week, it is possible to move them to the main tank.
  • Larvae need to be fed 3 times.
  • Water quality during this period is of paramount importance. Cleaning mad water changes on a daily basis to remove toxic waste.

Threadfin Rainbowfish and Suitable Tankmates

Avoid larger, boisterous, or nippy fish with them. Threadfin Rainbows are small and timid fish, they cannot compete with other species and might be targeted for bullying.

  • Fish

These small fish can be kept only with fish that is not big enough to eat or harass them such as Pygmy Cory, Boraras, Panda Garra, Clown Killifish, Guppies, Harlequin Rasboras, Otocinclus, Medaka Ricefish, etc.

Some tank mates to avoid are Cichlids, Goldfish, Monos & Scats, Jack Dempsey, Oscars, etc. Even Betta, Paradise fish, Siamese Algae Eater, Angelfish should never be placed in a tank with Threadfin Rainbows.

Many aquarists also ask questions about the compatibility of Threadfin Rainbows and Betta. The fact is that there can never be a blueprint that guarantees success. It is very individual. You can have a very aggressive Betta that will harass your Threadfin Rainbows. At the same time, Betta likes calm and Threadfin Rainbows enjoy movement. It may also stress the Betta. All in all, I would say that these species are not compatible.

  • Shrimp:

Threadfin Rainbows can be kept with dwarf shrimp. They have very small mouths to be a threat to adults or even juvenile shrimp.

However, newly-hatched shrimp are tiny enough to fit in their mouth. So, there is always a chance that they might snack on them.

Note: Shrimplets that are 1 – 2 weeks old will be too big for them to eat.

  • Snails:

They are compatible with any freshwater snail


  • Large and/or aggressive, and/or boisterous fishes.
  • Keep them away from all types of crayfish and most types of freshwater crabs. 

Related article:

In Conclusion

Threadfin rainbow (Iriatherina werneri) are gorgeous, easy to care for fish species, once you understand their needs and establish a routine.

With their delicate, long fins, they look really nice in a group and can be a really nice addition to a peaceful community tank.

These fish are very active. So, if you were looking for something that will really shine in the tank, then this is the fish for you. 


  1. Trappett, Andrew, Catriona H. Condon, Craig White, Phil Matthews, and Robbie S. Wilson. “Extravagant ornaments of male threadfin rainbowfish (Iriatherina werneri) are not costly for swimming.” Functional Ecology27, no. 4 (2013): 1034-1041.
  2. Herjayanto, Muh, Odang Carman, and Dinar Tri Soelistyowati. “Embriogenesis, perkembangan larva dan viabilitas reproduksi ikan pelangi Iriatherina werneri Meinken, 1974 pada kondisi laboratorium.” Akuatika Indonesia2, no. 1 (2017): 1-10.
  3. Nurkhasanah, Anna. “Maskulinisasi Ikan Pelangi (Iriatherina Werneri) Melalui Perendaman Embrio Dalam Ekstrak Tanaman Purwoceng (Pimpinella Alpina).” (2015).
  4. Herjayanto, Muh, Odang Carman, and Dinar Tri Soelistyowati. “Spawning behavior, female reproductive potential and breeding technique optimize of threadfin rainbowfish Iriatherina werneri.” Jurnal Iktiologi Indonesia16, no. 2 (2016): 171-183.
  5. Rakhmawati, Wulan Nurindah. “Nisbah kelamin ikan pelangi Iriatherina werneri pada perbedaan suhu pemeliharaan.” (2015).
  6. ROBERT, TYSON R. “An Ichthyological Survey of the Fly River in Papua New Guinea with Descriptions of New Species.”
  7. Adrian R. Tappim “Iriatherina werneri-aquarian review”. Fishes of Sahyl vol.2, N 3, 1985 г.
  8. Barry Crockford “Iriatherina werneri-breeding”. Fishes of Sahyl vol.2 N 3, 1985 г.
  9. Meinken, Hermann & W. Foersch (1975), “Iriatherina werneri, a new atherinid fish from New Guinea,” in Tropical Fish Hobbyist, volume 4, number 75.
  10. Allen, Gerald R. (1980), “A Generic Classification of the Rainbowfishes (Family Melanotaeniidae),” Records of the Western Australian Museum, 1980.

4 thoughts on “Threadfin Rainbowfish – Detailed Guide: Care, Diet, and Breeding

    1. Hi Dilala,
      Thank you 🙂
      Best regards,

  1. Hi Michael, great article!!
    I found the information on the temperature effect on the male percentage egg hatching to be particularly useful (I know that temperature affects the ratio, but I’ve never seen the actual breakdown before). I was looking for that information when I found your article.
    In Australia, these fish are also found in the Northern Territory (top centre of Australia).
    This NT location is the form that I am breeding.
    With this form, I recently won grand champion fish of the Brisbane Exhibition Show, beating some extremely expensive discus and very large and colourful snakehead gudgeons!
    The werneri from the Northern Territory have more red and black, while the werneri from Cape York are more yellow/green (and lack the red).
    I wouldn’t call them egg scatterers as they don’t scatter them on the bottom. As you say, their eggs are adhesive, and they normally lay them on plants. I collect the eggs on woollen “mops” to place in the hatching tank.
    I find the easiest way to rear young fish is in a tub of green water. It’s like hatching them into a liquid smorgasbord.
    Your rules for rearing section has an incomplete statement for the first dot point: avoid potential ? I assume predation?
    I generally keep them as a single species tank, but in nature they are also found with Pseudomugil gertrudae (spotted blue-eyes) and blue-eyes are one of the few types of fish that I would keep them with in a community tank.
    Strangely, I find I have more success with breeding if I add a small amount of salt to their tank. I don’t know why this works, as I’ve never found them in a saline or brackish environment.

    1. Hi Leo O’Reilly,

      Thank you for your kind words. Due to a significant amount of spam, the website has a strict filter. Unfortunately, it occasionally doesn’t allow normal comments through. However, I usually manually review and restore them from the spam section.

      The concept of “egg scatterers” is somewhat broader than it may initially seem, encompassing both those that simply scatter eggs and those that subsequently attach them. In essence, all non-livebearing fish are considered egg scatterers. Just as an aside.

      Regarding the incomplete sentence, you are correct – it should have been “predation and cannibalization.”

      How much salt do you add? It’s quite likely that it contributes to both combating various pathogens and aiding in osmoregulation.

      And of course, congratulations on your victory at the Brisbane Exhibition Show!

      Best regards,

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