Ricefish from the Genus Oryzias | Tropical Fish Hobbyist Magazine

Issue: August 2013

rice fish

The Ricefish: An Odd and Interesting Group

Author: Mike Hellweg

Often dismissed as being dull or drab, ricefish can truly shine if placed in a planted aquarium. These diminutive, peaceful schooling fish make the perfect addition to a calm community tank and are a terrific alternative to tetras.

I’m always amazed when I talk to someone who says that they’re bored with the aquarium hobby. They’ve kept “all” kinds of fish and there’s nothing new to keep. How far that is from the truth! Given the limited selection available locally to some people, one might overlook some really interesting fish. The group of fish we call ricefish falls into that category. They might be overlooked in a shop as small and plain looking, but when taken home and given a planted tank, their subtle beauty quickly becomes apparent. Then their behavior comes out and the fascination begins.


The genus Oryzias is an interesting and widespread group of just under three dozen species of small fish, ranging from India through Southeast Asia to Korea and across the sea to Japan and through the islands of the Philippines, Malaysia, and Indonesia. They are usually found in fresh water, though many species can be found in brackish and even marine environments—moving freely between them.

They are not closely related to killifish but are small, schooling fish that spend much of their time near the surface feeding on insects—filling the same niche as killies do in their respective habitats. This similarity has led many killie keepers to also seek out and work with the various species.


They are called ricefish because they are often found in the shallow flooded rice paddies throughout their range. This is an ideal habitat for many small species of fish—plenty of warm water, a lack of predators, lots of places to breed, and lots of food in the form of small insects and their larvae.


I’m sure many of you are familiar with the best-known ricefish, O. latipes, usually called the Medaka. Medakas probably originated in Japan, but are now found in Japan, China, and Korea. This is likely due to its popularity as a pet, which is recorded as far back as the late 17th century. It has been bred into a beautiful golden morph that is more commonly available today than the original creamy-white fish.

Scientific Uses

They are often used in scientific research and have even gone into space, where they have the distinction of being the first vertebrate to mate and produce healthy young in space. A group of them currently resides in an aquarium on the International Space Station. They are also at the forefront of genetic research, and a transgenic green, glowing variant is available to hobbyists in some countries. Along with the zebra danio (Danio rerio), O. latipes is likely the most widely studied vertebrate species in the world.

In the Hobby

Most hobbyists seeing ricefish in a shop would dismiss them offhand as being too drab; in truth, they are anything but! And even the drabbest of species has stunning bright-blue eyes that can draw a viewer’s attention from across the room. Many of the ricefish are very attractively patterned, and a few, such as the recently described neon ricefish (O. woworae), are downright spectacular.

All of the species are small with most topping out at less than 2 inches. Several, like the diminutive Mekong ricefish (O. mekongensis), are downright tiny at just about an inch or even less. The real giants of the genus seem to be concentrated on the island of Sulawesi, where there are several species that top 2 inches and one, the yellow-fin ricefish (O. profundicola), that reaches nearly 2½ inches.

Aquarium Care

Since many species are found distributed over a wide area, often with wide expanses of saltwater in between their freshwater habitats, ricefish are very adaptable. For example the Medaka (O. latipes) is found in Japan, Korea, and China, although recent studies are suggesting that what was thought of as one species (O. latipes) may actually be a species complex of several species. The Javanese ricefish (O. javanicus) is found from the island of Java all the way up to Thailand!

Water Requirements

This is great for hobbyists, as they usually can adapt to local tap water with little more being done than removing the chlorine or chloramines. We can instead concentrate on doing large, regular water changes to keep the dissolved pollutants to a minimum. Many killie hobbyists keep them in smaller tanks with clumps of plants like Java moss or Java fern. Often they are kept without even a filter in the tank!

As long as you feed live foods and do large, regular water changes, this isn’t a problem. They will thrive and even breed when kept in pairs in tanks as small as one gallon. These smaller tanks make necessary care, like water changes, pretty easy. The biggest thing to remember in small tanks is to stock lightly and keep the water clean. It is hard to regulate temperature in small tanks, but fortunately, most species are not too picky when it comes to temperature, either. As long as you are comfortable in the room, it is likely they will be as well. Temperatures in the 70s seem to be ideal for most species.

Cover Your Tank

One thing that is important to remember is that ricefish are excellent jumpers, so the tank has to be kept covered. If you want to use a small sponge filter in the tank for biological filtration and aeration, it is a good idea to drill a ¼-inch hole in the lid, if it is not equipped with a punch out, and run the airline tubing through this hole. A glass top can be notched at one corner for the airline, and then any excess open space can be covered with packing tape.

Community Life

They do well in small-fish community tanks, where they are model residents. Provide them with plants around the perimeter and a large open swimming area in the front. Most often they are found in areas where the current is slow to nonexistent, so they should be kept in tanks without strong filtration. A canister or sponge filter is ideal. In such a tank, a group of a half dozen or more will spend most of their time schooling out front and center, watching for their next meal.


In the wild, ricefish are omnivores, consuming everything from biofilm to insects to fish eggs, whatever is edible and most prevalent in the area while they are feeding. Anecdotal reports from friends in Japan say that even wild fish will take flake food sprinkled on the water surface! Modern high-quality flake and pellet foods are a perfect diet for them in the aquarium. If you want to breed them, however, it is important to improve this diet just a bit by offering live foods, such as daphnia, newly hatched brine shrimp, fruit flies, and/or Grindal worms, for several days. Other live foods, if available, are greedily taken as well.


Adults of most species are easy to sex. Females are generally a bit larger than the males. Males are generally a bit to a lot more colorful, depending on the species. In many species, the males have extensions on their anal fins, which can become spectacular in some species. Females generally have a much fuller body, while that of the males is usually much more slender. A close look at the area right in front of the anal fin will reveal a small pointed bump in the males—this is the genital papilla. In females, this area is usually fairly flat with only a small, rounded protrusion. And many times, even in a community tank, females can be seen swimming around with clusters of eggs in the morning, which is one of the most interesting things about the ricefish.

Reproductive Behavior

Scientists are still studying their reproductive behavior, and in many species, it is still unobserved. In those that have been described and observed, males have a single, pointed, tubular genital papilla and females have double, flattened, rounded, lobular papillae outside the urogenital pore. In addition, in some species the first couple rays of the anal fin are thickened and thought to be used in mating, though that has yet to be proven. In other species, males have small contact organs on the middle and back rays of the anal fin that are also used in mating. Males fertilize the eggs internally in most, if not all, species of Oryzias.


Mating occurs almost daily for several months, then the fish rest for a few months before beginning again. The fertilized eggs develop inside the female for several days. Each day, she lays up to 20 eggs early in the morning. The eggs have thin filaments that keep the eggs clustered near the female’s vent for a period of time, usually just a few hours, making her look as though she is swimming with a cluster of grapes attached. She swims through fine-leaved plants and the eggs brush off, attaching to the plants or falling to the bottom. From then on, she provides no care to the developing embryos. Depending on the species, the eggs hatch in a few days to as much as two weeks after being deposited on the plants.

In a few open-water species, such as O. sarasinorum and O. eversi, which are found in lakes and streams on Sulawesi, the female has extended pelvic fins that form a protective pouch in front of and around the developing eggs and she actually carries the eggs until they hatch. This can be up to 18 or so days after they are laid. In these species, the female’s body actually forms a notch behind the abdominal cavity and in front of the anal fin to help hold the eggs in place.

It’s amazing to me that the eggs of the diminutive O. mekongensis and the relatively large O. celebensis are just about the same size! As more species are studied, it wouldn’t be surprising to see more interesting reproductive habits appear in this group.


Well-fed adults rarely prey upon fry, but larger siblings might consider their younger siblings as a tasty morsel, so it’s a good idea to separate fry with more than a week’s difference in age. Fry are ready to feed upon hatching, and good first foods include items such as vinegar eels and microworms. The fry of many species will also take fine-powdered prepared fry diets.

Growth is rapid, and they will be able to take newly hatched brine shrimp by the end of their first week. Many species are ready to spawn for the first time when they are barely three months old. Considering that they live for only a year or so in the wild, this is likely an adaptation to both a short lifespan and being near the bottom of the food chain. In our aquaria, where they live well-fed, sheltered lives, they can often live for two or three years and some of the larger species can live as long as five years.

How I Breed Them

I maintain my specimens in mixed-species tanks for most of the year. I feed them daily with a high-quality flake or floating micropellet food. Several times a week, I add some live foods to the tank—daphnia, newly hatched brine shrimp, various worms, flour beetles, etc. When I want to get a spawn, I move from one to several pairs to a 5- or 10-gallon tank. For the smaller species, a 3-gallon critter tank works just fine. Whatever sized tank you choose, remember the lid! I add several clumps of fine-leaved plants or a couple of spawning mops and feed the adults heavily with live foods.

Water and Filtration

I usually add a small sponge filter to the tank, too, though this isn’t strictly necessary, and sometimes I’ll just add a gently bubbling airline to the tank. I change the water every other day by simply pouring about half of it out through the slots in the lid and refilling. I feed live foods like daphnia and several types of small worms two or three times a day. After two weeks, I move the adults back to the main tank.

Eggs and Fry

Over the next several days the eggs will begin to hatch. The tiny fry will be found right at the surface. The fry need to be fed right from the start. I leave daphnia in the tank all the time, and the fry can eat the daphnia nauplii as they are born as a supplemental food. They can be fed vinegar eels and microworms right from the start as well. As I mentioned earlier, they grow quickly and can take newly hatched brine shrimp by the end of their first week. The fry grow fairly quickly, and after two or three weeks, they can be over 3/8 inch long. At this time, I move them to a 10-gallon tank for growing out.

I don’t net fry. I slowly pour them from the hatching tank to this new tank. I always add a few small ramshorn snails to the tank at this point to help clean up any excess food. I don’t keep snails in the spawning tank, as they can eat the eggs. For the next couple of weeks, I do water changes every other day.

Moving Tanks

A good two-weeks’ spawn from three pairs can produce several hundred fry, so it is a good idea to keep moving them to larger tanks as they grow. After a few more weeks, they’ll be nearly ½ inch long, and then I move them to a 30-gallon breeder in my fishroom, where they will stay until they are ready to head out on their own. Surprisingly, for what some folks consider bland fish, they are almost always in demand and I have no trouble finding new homes for them.

Of course, I only raise a few dozen to a hundred or so of any given species, so I’m not flooding the market. One could theoretically raise thousands, but finding homes for so many would not be very practical. It’s much better to enjoy these fascinating fish, breeding just enough to keep the population going and maybe make enough to cover their maintenance and care.

A Pretty Little Fish

Though often passed over, once you stop and take a closer look, the members of the fascinating group of fish known as the ricefish make interesting additions to our aquarium hobby. A school of them in a community tank will draw the eye of even the most jaded viewer. Add in their ease of care, model behavior, and fascinating methods of reproduction and you have a winning group of fish for the aquarium hobby. The next time you’re in your local independent fish store looking for something new and different, don’t look any further than the fascinating ricefish. You’ll be glad you brought them home.

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