Danio Rerio – Detailed Guide: Care, Diet, and Breeding

Danio rerio, generally known as the Zebrafish are fascinating small freshwater fish that has been popular in the aquarium hobby for decades.

Zebrafish are known for being constantly active, bright, beautiful, hardy, readily acclimatizing to new environments, and easy to care for. Therefore, they are often recommended even for beginners.

Although Danio rerio does seem to require very little maintenance, they still need a bit more care than you may have been led to believe in pet stores. In addition, there is also a lot of conflicting information about this species on the Internet as well. Therefore, I feel obliged to add my voice and correct some of the falsehoods.

In this detailed guide, I have gathered all information about Danio rerio based on existing studies, research, experiments, and the experience of aquarists. You will know more about this remarkable fish including their behavior, feeding preferences, ideal tank requirements, and how to take good care of them.

Interesting fact: Zebrafish (Danio rerio) is not only popular in the fish-keeping hobby but it has been estimated that more than 3250 institutes spread through 100 countries work with these fish. This species has become the key research model organism in genetics, developmental biology, neurophysiology, biomedicine, and toxicology due to its high fecundity, low maintenance cost, transparent embryos, and rapid development.

Quick Notes about Danio Rerio

Name	Zebrafish
Other Names	Zebra danio, Rerio, Striped danio, Zebra, Leopard danio and Longfin danio
Scientific Name	Danio rerio
Tank size (minimum)	15 gallons (~60 liters) long
Keeping	Easy – Medium
Breeding	Easy – Medium
Size	1.2 – 1.6 inches (3 – 4 cm)
Optimal Temperature	75 – 82°F (24 – 28°C)
Optimal PH	7.0 – 8.0
Optimal GH	10 – 20
Dwellers	mostly mid-dwelling
Nitrate	Less than 80
Diet	Carnivore / omnivore
Temperament	Peaceful
Life span	up to 3 years
Color Form	Striped with dark blue, gold, and iridescent

Taxonomy and Etymology of Danio Rerio

Taxonomy: In 1822, Danio rerio was originally described by Francis Hamilton in his study “An Account of the Fishes Found in the River Ganges and its Branches”.

This species belongs to the family of freshwater fishes Cyprinidae, the most species rich vertebrate family. Currently, there are approximately 30 species of danios and another 40 species of Giant danios, many of which are popular in the aquarium hobby.

Etymology: The name Danio has Bengali roots from the name ‘Dhani’ (ধানি ), meaning ‘Of the rice field’.

The name ‘Rerio’ is presumably derived from a local name in Bengali dialects for the species.

Distribution of Danio Rerio

Danio rerio is indigenous to South Asia where it is widely distributed over the Indian subcontinent. The Zebrafish can be found in:

• the Ganges and Brahmaputra river basins in north-eastern India,
• Bangladesh and Nepal,
• in the Krishna river basin,
• in the states of Rajasthan, Gujarat, and Andra Pradesh (river basins draining into the Arabian Sea),
• Northern Myanmar and Sri Lanka.

Note: This geographic region has a monsoon climate, with pronounced rainy and dry seasons that have a profound effect on habitat parameters, including water chemistry and resource abundance.

Habitat of Danio Rerio

The Zebrafish have been reported to occur in a wide variety of habitat types, including ponds, pools, channels, irrigation ditches, paddy (rice) fields, and the edges of streams and rivers.

These fish prefer shallow (water depth range 4 – 15 inches or 10– 40 cm), still or slow-flowing waters (up to 0.3 ft/s or 10 cm/s) in unshaded locations with some overhanging vegetation.

Note: Danio rerio is also reported to be found inhabiting fast rivers and hill streams. However, some researchers strongly believe that this inconsistency in habitat preference probably results from the taxonomic confusion between genera Danio and Devario.

Description of Danio Rerio

Zebrafish are a small freshwater species. The average size of the adult fish is only about 1.2 inches (or 3 cm) from the tip of the snout to the origin of the caudal fin. They rarely exceed 1.6 inches (4 cm) long.

Distinguishing characteristics of Zebrafish:

• Spindle-shaped and laterally compressed body shape,
• The mouth is oblique and directed upwards,
• The jaw has mandibular prognathism (protruding the lower jaw), 
• Eyes are positioned on the side of their head and not visible from above,
• The incomplete lateral line extending to the pelvic fin base,
• Two pairs of barbells,
• Five to seven dark blue longitudinal stripes extending from behind the operculum into the caudal fin,
• Color pattern includes three types of pigment cell: dark blue, gold, and iridescent,
• The anal fin is also striped whereas the dorsal fin has a dark-blue upper edge that is bordered with white.

Interesting facts:

1. Zebrafish color can change in response to stimuli. They fade in color in response to light intensity (to camouflage) and darken during aggressive displays.
2. These fish exhibit a remarkable ability for regeneration even in adult stages. For example, they can completely regenerate the amputated caudal fin within a few weeks.

Variants and Color morphs

• The Leopard danio, GloFish, etc.

The Leopard danio and Glofish are not separate species. There is no neither molecular or morphological difference between these fish.

This mutation displays a spotted color pattern instead of stripes. Nowadays, Leopard danio color morph is mostly bred for the aquarium trade but also can be found in nature.

According to the study, the Leopard danio is now known to be a spontaneous mutation of the wild-type Danio rerio color pattern, with homozygotes displaying a spotted pattern, while heterozygotes have a disrupted stripe pattern.

• The Longfin Danio

The Longfin danio is another mutation resulting in elongated fins.

The Leopard danio, Longfin danio, and Zebra danio belong to the same species (Danio rerio). Therefore, they can breed and produce fertile progeny.

Lifespan of Danio Rerio

According to the study, in captivity, documented mean life spans for Danio rerio exceeded 3.5 years. The oldest individual survived 5.5 years.

Despite all these impressive results, Zebrafish appears to be primarily an annual species in the wild.

The point is that researchers noticed spinal curvature (caused by muscle degeneration) in domesticated and wild Zebrafish after their 2nd year in captivity

Spinal curvature is commonly associated with senescence and is not observed in a wild Zebrafish population. Obviously, fish die before this condition even develops.

Typical Behavior of Danio Rerio

Understanding their behavior is one of the most important aspects of caring for Zebrafish. It will help you create an optimal environment for them and reduce potential aggression against conspecifics and other tankmates.

Sociality:

Like most small species, Danio rerio is a highly social species (schooling fish). They form a tight group in which fish swim together in close proximity. In nature, this behavioral strategy provides multiple benefits, including defense against predators, access to mates, and efficient foraging.

The shoaling behavior is innate and starts soon after hatching. In the aquarium, you need at least 6 (bare minimum) of them to start. Large groups make them happier.

Interesting fact: Zebrafish use olfactory cues in kin recognition.

Important: Do not underestimate the importance of this factor because this is one of the reasons why Zebrafish become aggressive towards other fish and each other.  DO NOT keep them in too small numbers with other types of fish.

Activity:

Zebrafish are not shy or timid fish. They are moving almost all the time and moving very quickly. These fish are truly fun to watch.

They are also very playful fish. Unfortunately, they play really rough. It may even seem like they are fighting. However, they will often confine their aggressive “playing” to themselves, when they are kept in a large enough school.

Unfortunately, this extraordinary enthusiasm can stress other tankmates, sicken them or even lead to death.

Zebrafish may also start nipping at other fish to get them to play, especially, if kept in schools of limited numbers.

They prefer light to dark during the day. This is a diurnal species. They 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.

Dominance:

In the company of their own, Zebrafish establish dominance hierarchies of both sexes. Thus, they can show aggression against each other at some point.

Aggressive behavior includes chasing and in some cases even biting. You can also notice it when their fins are splayed and they slowly are circling one another while ascending to the top. It is a sign that they are irritated and ready to fight.

In addition, aggressive (dominant) fish are usually darker and exhibit more intense colors while subordinates are paler.

Nonetheless, once dominance relationships are formed, their aggression becomes less intense.

Notes:

• Zebrafish have relatively stable dominance hierarchies. Experiments showed that fish separated for 4 – 5 days have been shown to re-establish the same dominance ranks once reunited.
• Gender is not an important factor in determining the dominance rank.
• There is some controversy about whether the size of a fish plays a role or not. Some researchers registered that size correlated positively with the dominance rank, while others found that it was not.

Aggression:

Generally, Zebrafish are quite peaceful. However, some individuals (mostly males) may exhibit territoriality and antagonistic behavior.

This species is fin-nippers but it is rarely malevolent. They mostly do it as a play.

Signs of Alarm behavior:

• increase in shoal cohesion,
• agitated swimming or freezing on the substrate,
• decrease in feeding rate
• increase in aggression (including nipping).

Stimuli response:

Zebrafish on a black background often tend to be darker than fish on a light background. This is a natural reaction to camouflage from predators. In addition, stressed and scared fish will quickly lose their color and become pale.

Therefore, if you put them into the tank with a white substrate and light background or aggressive tankmates, their color will fade away.

Learning:

Zebrafish are relatively smart for their small size. It can learn and remember for long periods of time.

In one of the experiments, adults Danio rerio were trained to swim alternately to one or the other side of a divided tank to receive a food reward. Once trained, they could remember the task after a 10-day period.

It was also found that their learning ability depends on their age. Juveniles of 6 – 8 weeks old learned the task as well or even better than adults, whereas those of 3 – 4 weeks were not able to do so.

Interesting facts:

1. In Zebrafish, the preference for particular color patterns should be learned. Cross-reared fish prefer to associate with the color pattern with which they were raised.
2. They learn by visual and olfactory cues. It helps them to recognize and differentiate between familiar and unfamiliar conspecifics. Laboratory experiments have shown that Zebrafish display stress reactions in response to both visual and olfactory cues associated with predators.

Features:

• Social: Yes (very)
• Active: Yes (very)
• Peaceful: Yes (in large groups)
• Nippers: Yes

Diet of Danio Rerio

The Zebrafish can be classified as opportunistic omnivores with a strong preference for high-protein food.

Analysis of their gut content showed that they primarily feed on aquatic invertebrates (such as zooplankton and aquatic invertebrates). However, it has also been reported that filamentous algae, aquatic plant material, phytoplankton, invertebrate eggs, fish scales, spores, detritus, sand, and even mud were found in their stomachs.

In the aquarium, Danio rerio should be fed live prey items or some mixture with artificial diets. Ideally, their diet can include:

• brine shrimp,
• artemia nauplii,
• rotifers,
• daphnia,
• bloodworms,
• blackworms,
• fruit flies,
• grindal worms,
• cyclops, etc.

Giving them a diet based on live food items is also preferable because live diets (such as Artemia and rotifers) have balanced nutritional profiles for adult and young Zebrafish. Large prey (Bloodworms, grindal worms, etc,) is a more appropriate choice for adult Zebrafish. It is not recommended to feed Zebrafish an exclusively artificial diet (100 microns for larvae to 300/400 microns for adult fish)) as they exhibited slightly reduced, growth and survival rates compared to Artemia fed alone.

Zebrafish are mid-dwellers and mostly feed in the water column. Although the presence of terrestrial insects, inorganic elements, and detritus suggests they are also able to feed at the surface and on the substrate, this is not their primal way.

How Often to Feed Zebrafish?

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

Danio rerio is a small-bodied and very active fish with a fast metabolism. If it is possible, it would be better to give them small but frequent meals throughout the day to promote maximum growth.

Keep in mind that Zebra danios are very aggressive eaters.

Note: Anyway, do not be afraid to fast adults for a day or two. This is a pretty mild food deprivation, given that Zebrafish can stay healthy without food for a few days quite easily.

How much to Feed Zebrafish?

Generally, there are two main approaches utilized for that:

1. feeding to satiation (the so-called “five-minute rule”),
2. body weight feeding.

The “five-minute rule”, is the easiest and most popular (although not optimal) way to feed Zebra danios. This technique means that fish should consume all the food within 5-7 minutes.

Feeding by body weight means that you need to provide a ration as a fixed percentage of fish body weight each day. Adult fish should get food up to 10% of their body weight. Larval fish require up to 50–300% of their body weight each day. That is why it is recommended to feed juveniles multiple times a day.

Important: Remember – it is better to underfeed than overfeed them. Overfeeding is a major problem in maintaining water quality. Overfeeding deteriorates water quality, causes infections, and diseases and in general, puts them in danger of serious health conditions. Non-eaten particles of food (leftovers) and waste should be siphoned out after every feeding. It will help to keep the water clean and prevent contamination.

Features:

• Diet Type: Carnivore / omnivore
• Food Preference: Meat
• Feeding Frequency: Daily

Are Danio Rerio Plants Safe?

Yes, Zebrafish are completely plant safe. They will not eat any healthy plants in the tank. They simply do not eat healthy, living plant material.

Keeping and Caring for Danio Rerio

Zebrafish are usually straightforward and easy to care for if you know their requirements. However, for optimal results, here are some handy tips that you should follow in order to keep them in a tank.

Important: DO NOT add fish to the tank if it is not cycled yet. Although, for the most part, Zebrafish are pretty tough and hardy but high ammonia and nitrates will kill them.

Tank size:

Although Danio rerio is a small fish, they need a lot of space to move in the tank. They are very fast and active! Therefore, these fish are not suitable for nano tanks.

Technically, it is possible to keep a maximum of 5 – 6 Danios in a 10-gallon tank (40 liters). However, you will not see their true behavior and expression in such tanks. Therefore, I would not recommend it.

In my opinion, a group of 6 – 8 Zebrafish needs at least a 15-gallon (60 liters) long or even larger to show their true personality. 

Note: in laboratory experiments, Danio rerio is often housed in larger groups, for example, 15 – 16 or even 20–30 fish per 10-gallon tank (40-L tank). Nonetheless, keep in mind that these experiments have purposes other than ours and may conflict with their natural preferences.

Water Parameters:

Temperature: Zebrafish have a very high tolerance for wide temperature ranges. In their natural habitat, they can survive temperatures from near 43°F in winter to 100°F in summer (6 – 38°C). In the aquarium, they 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: While Zebrafish can also tolerate a wider pH range of 6.5– 9.0, they will be more comfortable in neutral to alkaline water. The ideal pH range should be between 7.0 – 8.0.

Hardness: Zebrafish have been classified as a “hard water” species, these fish do not like soft water. According to the study, the highest mortalities occurred at the lowest (5.5°GH) and the highest (32.5°GH) tested levels. They will appreciate hard water between 10 – 20 GH. 

Salinity: Danio rerio is a freshwater species but can tolerate a small range of salinities (up to 1 ppt) that technically extend to brackish conditions.

Dissolved oxygen: Zebrafish have high metabolic rates and consume more oxygen per unit weight than larger fish. Oxygen levels should be maintained at or just under saturation (∼8 mg/L at 82°F (28.0°C)) to ensure the health of the fish.

Nitrogen: Laboratory experiments showed that excess of ammonia (0.02 ppm) and nitrites. (1 ppm) are typically toxic to Danio rerio. These fish can tolerate nitrates relatively well. However, prolonged exposure to nitrates (up to 100 ppm) will cause health problems.

Water Changes:

You need to change the water (20-30 %) on a weekly basis to control your nitrates and water quality. Any water that you add to the tank should also be at least dechlorinated. Dechlorinators are available at any pet store (for example, Seachem Prime (link to check the price on Amazon).

Note: If you are planning to keep shrimp in the tank with Zebrafish, keep in mind that dwarf shrimp do not like big water changes. It can cause them to have some molting problems.

Filtration and Water Flow:

The only tricky part here is that Zebrafish prefer relatively still or slow-moving water. Thus, choose filters where you can adjust the flow (with the lowest possible setting for your tank).

Also, keep in mind the size of the fish. Some filter intakes can suck this small fish in! You will need to use an extra pre-filter in the form of a sponge to prevent them from being sucked in.

Lighting:

No special requirements. 

Zebrafish have a distinct diurnal activity pattern, which is synchronized with the light/dark and feeding cycles. In addition, their spawning is influenced by photoperiod.

Substrate:

No special requirements if you are not planning to breed Zebrafish. In this case, the substrate should be adapted to the needs of plants in your tank.

However, for breeding purposes, gravel is generally more favorable as substrate. It will allow eggs to survive predation from their parents.

Decorations and Hiding Places:

The provision of decorations and hiding places, such as live plants, artificial plants, driftwood, rocks, etc. will reduce the potentially negative effects of aggressive behavior.

In their natural habitat, Zebrafish are generally associated with aquatic vegetation, they also showed a clear preference for spawning in sites with aquatic vegetation.

Acclimation:

Gradually introduce them to the fish tank. Sudden changes in habitat can harm them. Before putting Zebrafish into your tank do not forget to temperature acclimate them.

Give them time to acclimatize/quarantine before adding them into the main tank. Do not rush the process! Do it very slowly to prevent any unnecessary stress.

Breeding Danio Rerio

Zebrafish are asynchronous group spawners and egg scatterers. Although these fish can breed under a wide range of conditions, they still display complex breeding behavior that involves competition among both males and females, preference for mates, preference for substrates, etc.

Sexing:

Adult Zebrafish (Danio rerio) are sexually dimorphic. However, it can be really hard to see the difference. The sex of juveniles cannot be reliably distinguished without dissection.

1. Size. Females are generally larger than males.
2. Growth rate: Females grow faster than males.
3. Color: Males are generally darker in color than females. They also have more yellow coloration in the anal fin compared to females. Females have little, if any, gold on their undersides.
4. Abdomens. Females can be distinguished from males because of their bigger underbelly. Males are more slender and torpedo-shaped.
5. Genitals: Females have a small genital papilla in front of the anal fin.

Interesting fact: Every Zebrafish individual starts to develop as an immature female and future males must undergo a ‘gonadal transformation’ to produce functional testes at approximately 5-7 weeks post-hatching (0.4 – 0.4 inches or 1-1.5 cm total length).

Maturity:

Danio rerio reaches maturity after 75 days at 77 ºF (25.5 ºC) when females are around 1 inch (2.5 cm) long. Males reach reproductive maturity at approximately 0.9 inches (2.3 cm) long.

Spawning cycle:

In the laboratory, domesticated Zebra danio can breed all year round whereas in nature spawning is more seasonal.

Mating:

Females usually prefer the odor of larger and unrelated males. Matings are also determined by the dominant male excluding other males.

Spawning starts in the morning (within the first minute of exposure to light following darkness) and continues for about an hour.

Males release steroid pheromones into the water to induce ovulation in females. In their turn, after ovulation, females release hormones that prompt male mating behavior.

Interesting observation: It seems like female pheromones have the ability to suppress the reproduction of subordinate females. Further, dominant females also produce more eggs than subordinates.

Courtship Behavior:

Male chases a female everywhere she goes and nudges her flanks with his snout to lead her to a spawning site. Alternatively, he may circle around the female and swim back and forth between her and the spawning site.

Some male Zebrafish can become territorial during mating around potential sites of oviposition. That is because females are also choosy with respect to sites for oviposition. In choice experiments, females clearly preferred a gravel substrate to silt.

Fertilization is external. Male will swim over the eggs to release milt to fertilize them.

Note: Some articles say that Zebrafish are also loyal to their breeding partner and mate for life. Do not believe this nonsense. Males and females will change their mating partners with ease.

Eggs:

Females of this species can usually produce clutches between 150 – 190 eggs. There is a positive correlation between the size of females and the number of eggs. Large females can produce more than 700 eggs.

Note: According to the study, females produce eggs of higher quality when they are exposed to male pheromones for several days prior to spawning, than isolated females.

The egg diameter is approximately 0.7 mm, they are non-adhesive and demersal. Fertilized eggs are transparent.

Females release eggs directly over the substrate without prior preparation.

Zebrafish do not care for their eggs or hatchlings and leave the spawning grounds soon after fertilization.

Hatching:

Depending on the temperature, it takes only 2 – 3 days at 82ºF (28ºC) for the fertilized eggs to hatch.

The mean size of newly hatched Zebrafish juveniles is around 0.12 inches (3 mm). They are optically clear.

After hatching, larvae adhere to hard surfaces by means of specialized cells on the head. By day 5, they need to inflate their swim bladders by swallowing air at the water surface. After that, larvae are capable of swimming and maintaining their position within the water column.

The main period of rapid growth takes place during the first 3 months after hatching and completely stops by about 18 months. Under optimal conditions and feeding regimes, they can grow up to 0.7 inches (1.8 cm) in the first 45 days of development and 1 inch (2.5 cm) after one year.

Breeding Tank for Danio Rerio

A breeding tank should have the following attributes:

• Zebra danios need a separate spawning tank because after scattering their eggs all over the tank bottom, parents start eating them. To avoid this situation, fill the rearing tank with gravel or marbles. Some breeders also use a net placed on a frame at 1 inch (2.5 cm) high from the bottom of the tank to create a double floor and protect the eggs from being eaten.
• Remove parents to the main tank to avoid larvae cannibalization as well.
• You need lighting. Reproduction in Zebra danio is strongly regulated by photoperiod. They generally spawn at dawn.
• The temperature should be around 82°F (28°C). Low temperature is very bad for egg development.
• Feed live foods to the parents 2 – 3 times a day to induce breeding. It should be done for a period of 1 – 2 weeks prior to the spawning of Danio rerio.
• Larvae need to be fed from 4 – 5 days post-fertilization when they start swimming.
• Larvae must be fed with a mixture of ciliates, rotifers, and Artemia nauplii. Live prey is preferable, due to the fact that they are generally more nutritionally balanced, visually and chemically attractive, digestible, and distributed evenly in the water column than artificial diets.
• Water quality during this period is of paramount importance. Large water changes (30 – 50 %) on a daily basis to remove toxic waste that will negatively impact the growth and survival of larvae.
• The water change should include removing dead or diseased larvae and any other debris.
• No or very low water flow. Water flow will negatively impact on survival of the larvae because they will be forced to expend energy on maintaining their position in the water column. In addition, the water flow will make prey more difficult to capture.

How to instigate breeding?

In nature, Danio rerio spawns during periods of heavy rains. So, a small drop in water temperature and a slight increase in water level may instigate their breeding.

Danio Rerio and Suitable Tankmates

Zebrafish are generally peaceful fish when kept in large schools. However, there is always a chance that they become nippers in community tanks.

Fish:

It is absolutely not recommended to house them with slow fish, fish with long fins (such as Betta), and fish that cannot stand up for themselves. For example, Zebrafish are aggressive eaters and there will be a lot of fighting during feeding time.

Some potentially compatible fish tank mates include Harlequin Rasbora, Pygmy Cory, Boraras, Swordtails, Panda Garra, Neon Tetras, Dwarf Ember Barbs, Clown Killifish, Least Killifish, Guppies, Endlers, Medaka Ricefish, etc.

However, it is in your best interest to monitor their interaction to ensure nothing goes haywire.

Dwarf shrimp:

If you are planning to breed dwarf shrimp it will not be a good decision to keep them with Zebrafish. They will definitely feed on shrimplets! 

In addition, these fish are very active and playful. So, even adult shrimp can be killed during molting when they are soft and absolutely unprotected.

Snails:

Zebrafish usually do not mess with freshwater snails. However, they can nip off their antennae periodically.

Summary of Bad Tank Mates for Zebrafish: 

• Larger or aggressive fish.
• Timid, slow-moving, and fish with long fins.
• Any crayfish species.
• Any freshwater crab species.
• Dwarf frogs.

In Conclusion

Danio rerio is an extremely popular aquarium species even among beginner aquarists. These fish are small, social, cute, and very active. So, if you were looking for something to be constantly moving or playing, then Zebrafish is the pet for you.

In addition, these fish are able to tolerate a wide range of water parameters and temperatures, and readily accept all sorts of live or dried fish foods.

As long as Zebrafish have adequate water parameters, tank size, food, and safety from their tankmates they will make great additions to your tank.

References:

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