Breeding the Threadfin Rainbowfish

Author: John Robertson

Gorgeous, peaceful, and active, the threadfin rainbowfish (Iriatherina werneri) is a spectacular aquarium fish. However, it’s not the easiest species to breed. A master fish breeder took up the challenge and provides advice on how you can keep and breed this remarkable rainbowfish.

Radiating Glamor

The threadfin rainbowfish (Iriatherina werneri) is a spectacular little fish that does not immediately remind you of the hobby’s more common rainbowfishes of the genus Melanotaenia. It is smaller and more slender and has an unusual tail-down posture, rather like a penguin characin. When stationary, its colors are pretty rather than gaudy.

Yet the fins of the male threadfin and the flamboyant way it displays these set it apart from all other fishes. Like other rainbows (family Atherinidae), it has two dorsal fins, but again the similarity is limited. The first dorsal is large and rounded. The second dorsal, anal, and ventral fins have huge black filaments that trail behind, somewhat folded like a peacock’s tail, but which burst into radiating magnificence when two males meet or when they are courting females.

Beautiful Ballet

Although they seem to hang tail down quite a bit, rather nervously, they are also very active and their majestic displays, with stupendous fins spread to the splitting point, are accompanied by lightning-quick dashes across open water, parallel with their rivals. In pairs or trios, they display to each other with sudden twists and turns coordinated to fractions of a second, their glorious filaments exploding into view, then folding instantly, then displaying again just as quickly. Constantly unpredictable, starting, stopping, diving this way and that, it is literally an underwater ballet.

Not only is this one of the most enchanting displays available in the hobby, but it is also given freely. Kept in a small group with other small and inoffensive fishes, the threadfin rainbow is always displaying. This remarkable show is seen every day. The aquarium is always full of life, with action bursting in all directions.

Like birds of paradise, appropriately, the threadfin rainbow hails from central to southern New Guinea and also from northern Australia. Including fins and a sickle shaped tail, the male reaches about 2 inches long and the females rather shorter. The body is torpedo shaped with a pointed snout and small mouth.

The base color of the threadfin is gray with a bright golden eye and operculum, but the tiny scales are reflective, gleaming with blue, gold, and green as they dash about. In breeding condition, the male’s belly is orange and his lips black. His fins become reddish, and the anterior dorsal and curvy tail show edges of blue. A pale stripe along the back and down the forehead appears metallic green or bluish gray from different angles. This rainbow of color contrasts with the jet black of those enormous trailing rays of the unpaired fins.

I bought 14 young fish and housed them in a 16-gallon tank. It was densely furnished with Java moss and other plants at both ends, but I left plenty of swimming space in the center to fully appreciate the energy of the fishes. I kept them in rain water at about 80°F and fed them mainly baby brine shrimp. I mentioned the small mouth. They will eat other foods, including a little flake, but even adults struggle with all but the smallest of bloodworms or mosquito larvae.

No Girls Allowed!

With regular water changes, the threadfins developed quickly, and it was not long before I was convinced they were spawning. With constant displaying and couples diving into the Java moss, I waited patiently for fry, but none appeared. I removed them to another tank, but no fry resulted. I tried a variety of things for over a year until I realized I was missing something fundamental—females!

I laugh about it now because it’s quite easy to determine gender in this species. The girls are smaller and lack the extravagant fins. In my group of 14, I had only two that I thought were females because their filaments were shorter and less dark. After months without success, I was suspicious. Then I read that it is common to find single-sex groups in dealers’ tanks! Perhaps the commercial breeders deliberately supply them that way to discourage home breeding.

After some months of searching, I found a group of females and purchased four. I placed them in a separate tank to condition them, thinking the ardent attentions of 16 long-frustrated males may have been too much too soon. Two weeks later, I placed two males and two females each in two 16-gallon tanks set up as I described above but with woollen mops also added to the tank ends, high in the water.

To my surprise, there was not much frenzy. I had expected amazing displays of those huge fins, corralling the females towards the mops, but it seemed to be a quieter affair. Maybe this was because the adult males were getting rather old or because the real fireworks are reserved for rival males, as I had seen so many times before. Maybe I just missed the best of it when I was at work all day. Anyway, after a few days, I saw males sensually quivering among the mops and plants, filaments folded back, heads pointing rather up or down. This tempted the females in, and spawning took place in a rather gentle fashion.

The Tiniest of Fry

In only two weeks, a few tiny fry were spotted on the water surface, but they did not survive with the adults. I removed the woolen mops to a 12-inch tank with water from the breeding tank, and every day or so, a few more free-swimming fry would appear. I changed water twice a week, replacing with water from the parents’ tank.

They are among the tiniest fry I have seen and reputedly have the smallest of mouths. My research revealed that success requires feeding with microscopic foods in early stages. I started with green water from my wife’s greenhouse and the smallest of powdered foods (5 to 20 micron) until the fourth day when I added paramecia. I continued to feed this diet until the first young were 10 days old, when I replaced the green water with newly hatched brine shrimp. Only the largest of the fry showed any interest in the nauplii. They were three weeks old before I was sure they were all eating the brine shrimp, and at that time, I counted 18 fry.

Until the fry were five weeks old, they always seemed more excited by paramecia than by brine shrimp. The numbers of fry dwindled, though, and I finally raised only eight young from this brood and a further eight from a second group.

When the fry were five weeks old, I wrote to a friend that they were beginning to look like little glassy rainbowfish (rather like Popondetta). The largest ones had begun to hang diagonally, tail down as the adults do, but they did not have the pointed snout yet. At that stage, they are rather the size of newly born guppies.

From that point, they grew quickly and started to look like females at about seven weeks. The male fin extensions began to develop at 12 weeks, but the sexes were not easily distinguished for several weeks more.

Let the Rainbow Shine!

The threadfin rainbowfish is not the easiest fish to breed, but it is one of the most remarkable and rewarding small fish to keep in a group with other small and inoffensive fishes. If they are given space to demonstrate their amazing frenetic dances and display their stupendous fins, you can expect to witness one of nature’s most wonderful performances. After over 40 years in this hobby, I cannot think of anything more spectacular that the fish world could offer the owner of a 16-gallon aquarium!