The Slender Rasbora is a great fish for aquarists looking for a less common species to add interest to their tank!
The Slender Rasbora Rasbora daniconius is just as attractive as its flashier cousins even if it isn’t as colorful. In the wild, this is a good-sized cyprinid, usually reaching just shy of 6 inches (15 cm) in length. Some reports even say an occasional individual can reach almost 8 inches (20 cm)! In captivity, it doesn’t reach either of those sizes. An aquarium specimen will usually only reach a length of about 3 1/2 inches (8 – 9 cm).
To delightfully liven up the middle of an aquarium display, add a school of these subtle beauties. They have a silver body with olive undertones, but the scales are large and highly reflective, giving a sparkling effect. The belly of the males will be yellow or reddish while the female’s will be whiter. However, their most prominent feature is a blue-black line outlined in bright gold that runs the length of the body. This feature has inspired several rather descriptive names, including Striped Rasbora, Black Line Rasbora, Gold Line Rasbora, and Golden Striped Rasbora.
In the wild, they may be found swimming rivers in schools with hundreds of individuals, so in the aquarium, a school is a must. The companionship of their own species is absolutely essential to their well-being. Never keep fewer than 6 to 10 individuals, with more being even better. If this fish is kept singly, it will waste away. These fish do fine in community tanks but will suffer in the company of aggressive fish, so keep them with fish of similar size and temperament.
These fish are a good choice for beginning aquarists who want a tank of peaceful, schooling fish. The Slender Rasbora is robust and will readily accept aquarium foods. However, this cyprinid requires good quality water and a well-maintained aquarium. Fluctuations in water quality leave this fish weak and prone to disease.
They really look beautiful in a tank that simulates their natural habitat with plants, rocks, wood, and substrate. An aquarium best suited to this fish would be large and roomy with a fine gravel bottom and dense plantings along the edges. A school of these fish can be kept in a 20-gallon aquarium, but a larger tank is better for these lively fish and easier to maintain. They can jump with surprising gusto, so keep the aquarium tightly covered.
For Information on keeping freshwater fish, see:
Freshwater Aquarium Guide: Aquarium Setup and Care
- Kingdom: Animalia
- Phylum: Chordata
- Class: Actinopterygii
- Order: Cypriniformes
- Family: Cyprinidae
- Genus: Rasbora
- Species: daniconius
- Aquarist Experience Level: Beginner
- Size of fish – inches: 5.9 inches (15.01 cm)
- Minimum Tank Size: 20 gal (76 L)
- Temperament: Peaceful
- Aquarium Hardiness: Moderately hardy
- Temperature: 71.0 to 80.0° F (21.7 to 26.7° C)
- My Aquarium – Enter your aquarium to see if this fish is compatible!
Habitat: Distribution / Background
The Slender Rasbora Rasbora daniconius was described by Hamilton in 1822. They are found in South East Asia in the Mekong, Chao Phraya, and Salween basins and the northern Malay Peninsula, and to the west in the Indus River of Pakistan and south in Sri Lanka. It has also been found in Borneo and Sumatra.
This species is listed on the IUCN Red List as Least Concern (LC). It has a wide distribution, is adaptable to a variety of habitats, and lacks any known major widespread threats. Other common names it is known by include Striped Rasbora, Black Line Rasbora, Gold Line Rasbora, Blackline Rasbora, Common Rasbora, Golden Striped Rasbora, Slender Barb, and Striped Barb.
This fish is very similar in appearance to its close cousin Rasbora labiosa, which is also called a Slender rasbora. The R. labiosa was once considered to be a subspecies of R. daniconius but is now recognized as a distinct species. In earlier times, R. labiosa probably became more readily available to the trade as it was first cultivated in captivity in 1935. Though these two resemble each other closely, R. labiosa is actually only about half the size of R. daniconius. Its total length is about 3 3/4 inches (8.5 cm). There are some other distinctions, too. The R. labiosa is much more slender, almost cylindrical in shape, and the lateral line is reduced. It also has smaller fins and fat, fleshy lips.
These fish inhabit all sorts of waterways from ditches, canals, brooks, and streams to medium and large rivers as well as ponds and flooded fields. They are also found in brackish waters. However, they are primarily found in sandy streams and rivers with slow-moving, muddy waters where they tend to form large schools and feed on small aquatic insects and detritus.
- Scientific Name: Rasbora daniconius
- Social Grouping: Groups
- IUCN Red List: LC – Least Concern
The Slender rasbora is a slim fish, as the common name implies. It can reach a total length of just under 6 inches (15 cm) in length, with some reported at close to 8 inches (20 cm), but in the aquarium, it will commonly reach about 3 1/2 inches (8 – 9 cm) in length. They have a typical lifespan of 5 to 8 years with good care.
The general body color is silver with olive undertones. The belly will be whiter on males and yellow or reddish on females. These fish have a striking blue-black line finely outlined in gold that runs the whole body length, right through the tail. Scales are large and highly reflective, and fins are small and yellow.
- Size of fish – inches: 5.9 inches (15.01 cm) – It can reach up to 6″ (14 cm) in the wild but usually only reaches 3.5 inches (9 cm) in the home aquarium. An occasional specimen has also been reported at up to almost 8″ (20) in the wild.
- Lifespan: 5 years – They have an average lifespan of about 5 to 8 years years with proper care.
Fish Keeping Difficulty
This fish is a great choice for the beginning aquarist as it is robust and fairly hardy, but it must be kept in a school. With well filtered water and regular maintenance, these fish will do very well. They are usually not very picky eaters and will accept and thrive on quality flake foods, with the addition of occasional live foods.
- Aquarium Hardiness: Moderately hardy
- Aquarist Experience Level: Beginner
Foods and Feeding
The Slender Rasboras are omnivores. In the wild, they feed on small aquatic insects and detritus. In the aquarium, their diet should be based around a quality flake or pellet food. A good balance of prepared and live foods is required for optimal health. Daphnia, brine shrimp, and bloodworms all make excellent choices for live foods. Some green foods like lettuce are good, too. Dip a washed lettuce leaf in boiling water to soften it. Take care to remove the uneaten lettuce from the tank within a few hours. These fish will do best when offered food several times a day, but only offer what they can eat in 3 minutes or less at each feeding. If you feed only once per day, provide what they can eat in about 5 minutes.
- Diet Type: Omnivore
- Flake Food: Yes
- Tablet / Pellet: Yes
- Live foods (fishes, shrimps, worms): Some of Diet
- Vegetable Food: Some of Diet
- Meaty Food: Some of Diet
- Feeding Frequency: Several feedings per day – Offer only what they can consume in 3 minutes or less with multiple feedings per day.
These fish are not exceptionally difficult to care for provided their water is kept clean. At least 25 to 50% of the tank water should be replaced once a month. If the tank is densely stocked, 20 to 25% should be replaced weekly or every other week. The substrate should also be vacuumed during water changes to avoid accumulation of waste.
- Water Changes: Monthly – If the tank is densely stocked, water changes should be done every other week.
The Slender Rasbora is a schooling species that spends much of its time in the middle regions of the aquarium. They are very active swimmers, so they tend to do better in larger and longer aquariums where they can school. A school of 6 to 10 needs to be kept in a tank of 20 gallons or more. This rasbora is fairly hardy and will adapt to most aquarium conditions.
The key to making this fish extra beautiful is making it feel at home. Setting up a habitat that resembles their natural environment is the best way to do this. Their natural habitat provides a lot of direct sunlight, so place the aquarium where it will get natural sunlight. If this isn’t possible, get a high quality light that will simulate this. They prefer medium-hard, slightly acidic water conditions. These fish will also appreciate a reasonable water current, so use a good filter to create this effect. Keep the tank carefully covered as these fish are liable to jump if startled or excited.
Provide a sandy or fine gravel substrate with a few smooth rocks and pebbles. The tank must be densely planted with a good variety of plants to make them feel safe as well bring out their colors. These extremely active swimmers need open space to swim unobstructed. Have plantings reaching the surface of the aquarium around the sides and back with a few broad-leaved plants for shelter. With the right water chemistry for this fish, many aquatic plant species can flourish. Bogwood is also a great addition because it will release tannins and help get the water to the swampy and acidic conditions favored by this species. Aquarists who really want the natural feel could add some aquarium-sealed dried bamboo to simulate the reeds that grow along the shoreline of their natural habitat.
- Minimum Tank Size: 20 gal (76 L) – A school will do best in a large, long aquarium.
- Suitable for Nano Tank: Yes
- Substrate Type: Any
- Lighting Needs: Moderate – normal lighting
- Temperature: 71.0 to 80.0° F (21.7 to 26.7° C)
- Breeding Temperature: 80.0° F – Breeding temperatures need to be slightly higher than normal aquarium temperatures.
- Range ph: 6.0-7.5
- Hardness Range: 8 – 17 dGH
- Brackish: No
- Water Movement: Moderate
- Water Region: Middle – The Slender Rasbora is a mid-water fish.
These fish require a sizeable school of at least 6 to 10 individuals to display healthy coloration and behavior. They will have a pecking order but will never really cause harm to each other. The males will display their best colors as they compete for the attention of females.
These fish are peaceful but quite active. Although they occasionally quarrel, they are generally good natured and should be mixed with fish of similar temperament. Good tankmates include many popular community fish. Cyprinids are especially good as well as some of the larger characins and live bearers like mollies, plates, and swordtail fish. They also do well with bottom-dwelling, peaceful catfish and loaches, and even some of the dwarf cichlids.
- Temperament: Peaceful – This fish will occasionally quarrel with tankmates but without damage.
- Compatible with:
- Same species – conspecifics: Yes – This fish needs to be kept in a school of at least 6 to 10 individuals.
- Peaceful fish (): Safe
- Semi-Aggressive (): Threat
- Aggressive (): Threat
- Large Semi-Aggressive (): Threat
- Large Aggressive, Predatory (): Threat
- Shrimps, Crabs, Snails: Safe – not aggressive
- Plants: Safe
Sex: Sexual differences
The females are slightly plumper, stronger, and deeper bodied, and have a whitish belly. Males are more slender and smaller, with a yellowish or reddish belly.
Breeding / Reproduction
Slender Rasboras are relatively easy to breed with either pairs or in schools. Like other cyprinids, they exhibit no parental care for the young, but they differ slightly in their spawning method. Rather than being open water egg scatterers, their eggs are shed among the leaves of plants. Condition them with small offerings of live foods several times a day, usually for about 4 weeks. Conditioning well with live foods will also bring out the males’ color. When well fed, mature females should begin filling out with eggs.
A large tank (20+gallons) is required for spawning as part of the courtship is a very intense chase. The water should be medium hard at about 12° dGH. From their normal water conditions, slightly drop the pH (to slightly acidic at about 7.0 pH) and slightly increase the temperatures. Filtration isn’t really necessary, but you can add a small, air-powered sponge filter or some peat filtration. A pair will spawn on fine-leaved plants, so be sure to include Hornwort, Cabomba, Milfoil, or a similar species in the breeding tank. Java moss or Crystalwort (Riccia) also work if anchored down.
Courtship consists of displays and chasing by the male, with the female hiding among the plants until she is ready to spawn. Spawning will usually occur in the morning hours. They assume a side-by-side position with the male attempting to curl around the female. Eggs will then be shed among the plant leaves, and the female will find another hiding area for the next spawn.
After the spawn, remove the parents as they will eat the eggs. The fry will hatch in 3 to 4 days. The young are fairly large and become free-swimming in about 3 more days. Provide starter foods like infusoria for the first few days. The fry grow quickly and will soon be large enough to eat baby brine shrimp. See Fish Food for Fry for information about types of foods for raising the young.
- Ease of Breeding: Moderate
Slender Rasboras are very hardy, and disease is not usually a problem in a well-maintained aquarium.
Some diseases that are common in rasboras are dropsy, fin rot, and Ich if good water quality, nutrition, and maintenance is not provided. With any additions to a tank, such as new fish, plants, substrates, and decorations, there is a risk of introducing disease. Properly clean or quarantine anything that you want add to an established tank so as not to upset the balance.
These fish are very resilient, but knowing the signs of illness, and catching and treating them early, makes a huge difference. An outbreak of disease can often be limited to just one or a few fishes if dealt with at an early stage. The best way to proactively prevent disease is to give your fish the proper environment and a well-balanced diet. The more closely their environment resembles their natural habitat, the less stress the fish will have, making them healthier and happier. A stressed fish is more likely to acquire disease. For information about freshwater fish diseases and illnesses, see Aquarium Fish Diseases and Treatments.
The Slender Rasbora is difficult to find, but specialized fish stores may carry them. They are also sometimes available online and relatively inexpensive.
- Animal-World References: Freshwater Fish and Plants
- Rasbora daniconius (Hamilton, 1822) Slender rasbora, Fishbase.org
- Rasbora daniconius, The IUCN Red List of Threatened Species
- Dr. Rudiger Riehl, Hans A. Baensch, Aquarium Atlas Vol. 2, Publisher Hans A. Baensch, 1993
- Joseph S. Nelson, Fishes of the World, Wiley, 2006.
- David Alderton, Encyclopedia of Aquarium and Pond Fish , DK Publishing, Inc., 2005.
- Dr. Herbert R. Axelrod, Aquarium Fishes of the World, TFH Publications, 1998
- Dr. H.R. Axelrod, Dr. W.E. Burgess, Dr. C.W. Emmens, N. Pronek, J. G. Walls, R. Hunziker,
Dr. Axelrod’s Mini-Atlas of Aquarium Fishes, Mini- Edition, T.F.H. Publications, Inc., 1987
- Marc Ladonne, Aquarium Fish , Barnes Noble, 1999 (1996).
Featured Image Credit: Sudhakar Bisen, Shutterstock