Danio rerio — Zebrafish

Species Overview

Danio rerio, commonly known as the zebrafish or zebra danio, is a small freshwater cyprinid native to the slow-moving streams, rice paddies, and stagnant water bodies of South Asia, including India, Bangladesh, Nepal, and Myanmar. The species was first described by Francis Buchanan-Hamilton in 1822. In the aquarium hobby, it has been a staple for over a century due to its hardiness, active schooling behavior, and striking horizontal blue-and-gold stripes that extend from the gill cover to the caudal fin. Zebrafish are also one of the most important model organisms in developmental biology, genetics, and toxicology, which has led to a wealth of publicly available scientific data on their care and physiology. In the hobby, they serve as an excellent beginner fish and a reliable dither species for community tanks.

Tank Requirements

Minimum tank size: 10 gallons (38 liters) for a small school of 5–6 individuals; 20 gallons (75 liters) or larger is recommended for a group of 8–10 to allow natural shoaling behavior.

Substrate

Fine, dark-colored gravel or sand is ideal. Dark substrates enhance the contrast of the fish's stripes and reduce stress. Avoid sharp-edged substrates that could injure the barbels.

Decor

Provide open swimming areas with moderate planting along the sides and back. Live plants such as Java fern, Anubias, and Vallisneria are well-tolerated. Driftwood and smooth rocks can be added to create visual breaks. Zebrafish appreciate floating plants (e.g., Salvinia, duckweed) that diffuse overhead light and simulate their natural shaded habitats.

Tank Mates

Zebrafish are peaceful, active shoalers that do best in groups of 6 or more. Suitable tank mates include other small, non-aggressive species such as neon tetras, cherry barbs, corydoras catfish, and small rasboras. Avoid large or fin-nipping fish like tiger barbs or cichlids. Do not house with long-finned fish (e.g., bettas, fancy guppies) as zebrafish may nip at flowing fins.

Water Parameters

Parameter	Optimal Range	Notes
Temperature	64–78 °F (18–26 °C)	Breeding best at 78–82 °F (26–28 °C)
pH	6.5–7.5	Stable pH more important than exact value
General Hardness (GH)	5–19 dGH	Moderately hard water acceptable
Carbonate Hardness (KH)	3–10 dKH	Buffers pH swings
Ammonia (NH₃)	0 ppm	Toxic even at trace levels
Nitrite (NO₂⁻)	0 ppm	Equally toxic
Nitrate (NO₃⁻)	< 20 ppm	Ideally < 10 ppm for long-term health

Zebrafish are remarkably tolerant of suboptimal conditions but thrive and display best coloration in clean, well-maintained water. Weekly water changes of 25–30% are recommended.

Filtration and Lighting

Filter Type

A sponge filter, hang-on-back (HOB) filter, or internal power filter rated for at least 2–3 times the tank volume per hour is sufficient. Zebrafish are not strong swimmers but appreciate gentle to moderate water flow. Avoid powerful canister filters that create strong currents unless baffled.

Flow Preference

Low to moderate flow. In nature, they inhabit slow-moving waters. Excessive current can stress them and cause fin damage. Use a spray bar or diffuser if needed.

Lighting

Moderate lighting (0.5–1 watt per gallon LED or fluorescent) for 8–10 hours per day. Zebrafish do not require high light, but it supports plant growth. Provide shaded areas using floating plants or decor to allow fish to retreat from bright light.

Diet and Nutrition

Staple Food

High-quality flake food or micro-pellets formulated for tropical omnivores. Look for products with whole fish meal, spirulina, and added vitamins.

Treats

Live or frozen foods such as daphnia, brine shrimp (adult or nauplii), bloodworms, and microworms. Offer 2–3 times per week to enhance growth and color.

Feeding Frequency

Feed small amounts 2–3 times daily. Zebrafish are continuous grazers in the wild; multiple small meals prevent bloating and reduce waste.

Portion Rule

Give only what the fish can consume within 2 minutes per feeding. Overfeeding is the most common cause of water quality problems in zebrafish tanks.

Supplements

Zebrafish generally do not require additional supplements if fed a varied diet. However, for breeding or if using predominantly dry foods, consider:

• Spirulina powder – added to food 1–2 times per week to boost immune function and coloration.
• Garlic extract – can stimulate appetite and has mild antiparasitic properties.
• Vitamin C and E supplements – especially for fish in breeding condition; available as liquid additives for live foods.
• Calcium and iodine – important for fry development; can be provided via enriched Artemia nauplii.

Common Health Issues

1. White Spot Disease (Ichthyophthirius multifiliis)

Symptoms: Tiny white cysts on fins, body, and gills; flashing; rapid breathing. Treatment: Raise temperature gradually to 82–86 °F (28–30 °C), add aquarium salt (1–2 teaspoons per gallon), and use a commercial malachite green-based remedy for 5–7 days.

2. Fin Rot (Bacterial: Aeromonas, Pseudomonas)

Symptoms: Ragged, disintegrating fins with white or red edges; lethargy. Treatment: Improve water quality (0 ammonia/nitrite), add broad-spectrum antibiotic (e.g., erythromycin or tetracycline) per label instructions, and treat for 7–10 days.

3. Columnaris (Flavobacterium columnare)

Symptoms: Cotton-like patches on mouth, gills, or body; frayed fins; rapid death. Treatment: Isolate affected fish, use antibiotic bath (kanamycin or nitrofurazone), reduce temperature to 72–75 °F (22–24 °C), and add tannins (almond leaves) to inhibit bacterial growth.

4. Swim Bladder Disorder

Symptoms: Inability to maintain buoyancy; floating upside-down or sinking. Treatment: Fast for 24–48 hours, then feed a blanched, deshelled pea (for constipation) or treat with a commercial swim bladder remedy. Ensure water parameters are optimal.

5. Internal Parasites (e.g., Capillaria, Hexamita)

Symptoms: Weight loss despite eating; white, stringy feces; hollow belly. Treatment: Use a medicated food containing praziquantel or metronidazole for 5–7 days. Quarantine new fish for 2 weeks.

Beginner Mistakes

• Keeping fewer than 5 zebrafish: They are shoaling fish; solitary individuals become stressed, skittish, and prone to disease. Always maintain a group of 6 or more.
• Using a bowl or unfiltered container: Zebrafish produce significant waste; a cycled, filtered aquarium of at least 10 gallons is essential for stable water quality.
• Overfeeding: Uneaten food decays quickly, spiking ammonia and nitrate. Feed only what they can finish in 2 minutes, 2–3 times daily.
• Mixing with aggressive or fin-nipping species: Zebrafish are peaceful but may become targets or bullies themselves. Research tank mate compatibility before adding fish.

FAQ

How long do zebrafish live in an aquarium?

With proper care, zebrafish typically live 3–5 years. Some well-maintained individuals have been reported to reach 5.5 years in captivity.

Can zebrafish be kept in a 5-gallon tank?

A 5-gallon tank is too small for a proper school. Minimum tank size for a group of 5–6 zebrafish is 10 gallons. Smaller tanks lead to stunted growth, aggression, and water quality issues.

Do zebrafish need a heater?

Zebrafish are subtropical and can tolerate temperatures down to 60 °F (15 °C) for short periods, but a heater set to 72–78 °F (22–26 °C) ensures stable conditions, optimal metabolism, and disease resistance.

Why are my zebrafish losing their stripes?

Stripes can fade due to stress, poor water quality, or nutritional deficiency. Ensure water parameters are optimal, reduce tank lighting, and provide a varied diet with spirulina and live foods. Stripes usually return within 1–2 weeks after correction.

Are zebrafish good for beginners?

Yes. Zebrafish are among the hardiest and most forgiving freshwater fish, making them excellent for beginners. They are active, easy to feed, and adapt to a range of water conditions. However, they still require a cycled tank and regular maintenance.

References