Cardinal Tetra Care Guide: Complete Setup & Tank Mates

Quick Facts

• Common Names: Cardinal Tetra, Red Neon Tetra
• Scientific Name: Paracheirodon axelrodi
• Adult Size: 1.5-2.0 inches (3.8-5.0 cm)
• Lifespan: 4-5 years in captivity (up to 8 years with excellent care)
• Care Level: Beginner to Intermediate
• Temperament: Peaceful
• Diet: Omnivore
• Minimum Tank Size: 20 gallons for a proper school
• Temperature Range: 73-81°F (23-27°C); Optimal: 76-80°F (24-27°C)
• pH Range: 5.5-6.8 (optimal); tolerant of 3.5-7.5
• Water Hardness: 1-8 dGH optimal; very soft water (1-5 dGH) preferred for long-term health
• Breeding Difficulty: Difficult to Advanced
• Native Range: Amazon Basin—middle and upper Rio Negro, Orinoco watershed (Brazil, Venezuela, Colombia)

Introduction

Cardinal tetras are stunning little fish that look like they’re lit from within. Their electric blue stripe runs the full length of their body, paired with a deep red stripe that extends nearly to their tail. If you’re familiar with neon tetras, cardinals are their cousins—but with a full-body red stripe instead of just a partial one. They’re peaceful, colorful, and perfect for community aquariums, making them a favorite among both beginners and experienced aquarists. This guide will walk you through everything you need to know to keep them thriving.

Video Overview

Appearance

Cardinal tetras are sleek, torpedo-shaped fish with one of the most distinctive color patterns in the aquarium hobby. Their streamlined bodies are built for swimming through densely planted blackwater streams in their native Amazon habitat.

Body Structure: Small, compressed laterally (side to side), with a slight upward curve to the mouth. They have an adipose fin—a small, fleshy fin between their dorsal and tail fins—which is characteristic of their Characidae family.

Coloration: This is what makes cardinals so special. Imagine an electric blue stripe running nearly the full length of their body, complemented by a vivid red stripe along the lower half. The contrast between the bright iridescent blue (created by light-reflecting crystals in their skin) and the deep crimson red is truly stunning. The top of their body is olive to brownish, and their belly is silvery-white. Their fins are mostly transparent with just a hint of color at the edges.

Key trait: The full-length red stripe (extending from behind the head nearly to the tail) is the main difference from neon tetras, whose red stripe only covers the back half of their body. This seemingly small difference affects how light reflects and how the fish appears in different tank setups—something worth keeping in mind when designing your aquascape.

Sexual Dimorphism: Females are noticeably larger and rounder when carrying eggs, while males stay slim and streamlined. If you look closely at mature fish, males have a straighter blue stripe and a slightly hooked anal fin, while females have a more curved stripe following their plumper body shape.

Creating the Right Environment

Cardinal tetras show their best colors and behavior when you replicate their natural blackwater habitat. What does that mean practically? Think dim lighting, dark substrate, and plenty of plants—the kind of setup that might look moody to us but feels like home to them.

Dark Substrate Makes All the Difference: Use fine black or dark brown sand or small gravel. Against this dark background, the cardinal’s colors absolutely pop. You’ll see their electric blue stripe glow and their red stripe deepen. Beyond aesthetics, dark substrates replicate their natural leaf-litter-covered bottom and reduce stress, meaning more confident, visible fish.

Dense Plants with Open Swimming Space: Heavily plant the tank perimeters with tall stem plants like Hygrophila or Rotala, broad-leafed plants like Amazon Sword, and fine-leaved plants like Java Fern. Scatter floating plants to create shaded areas. But don’t turn the entire tank into a jungle—leave open midwater areas where they can school and forage naturally. The balance between cover and open space is what brings out their personality.

Tannin-Stained Water from Driftwood and Leaf Litter: Adding Malaysian driftwood, oak leaves, or Indian almond leaves serves multiple purposes: they acidify the water naturally (matching their preferred pH), release beneficial compounds with antibacterial properties, and create the amber-tinted water that cardinals evolved in. This isn’t just atmospheric—it actually reduces stress and improves long-term health. Start with a few leaves and adjust based on how much you want the water tinted.

Care Basics

Water: Cardinals need warm, soft, slightly acidic water. Maintain 76-80°F (ideally 78°F), pH 5.5-6.8, and 1-8 dGH (softer is better for long-term health). They can tolerate a wider range, but they truly thrive in these parameters. The key is stability—sudden changes stress them far more than the absolute values.

Tank Size: Start with 20 gallons minimum for a proper school of 10-15 fish. Smaller tanks (15 gallons) work for groups of 6-8, but larger tanks provide the water stability that cardinals depend on and allow for better behavior. If you’re adding tank mates (which you should—they’re community fish), 29 gallons or larger gives everyone space.

Food: Cardinals are omnivores and need variety. Use high-quality small flakes or micro-pellets as your staple (look for 40%+ protein). Supplement 3-4 times per week with frozen foods: baby brine shrimp, daphnia, or finely chopped bloodworms. Live foods like newly hatched brine shrimp are excellent for conditioning them before breeding and enhance their coloration. Feed once or twice daily in portions they can finish in 2-3 minutes. Remove uneaten food immediately—uneaten food degrades water quality quickly.

Equipment: You’ll need a reliable heater (100-150 watts for 20-40 gallons) because consistent temperature is non-negotiable. A sponge filter or gentle hang-on-back filter is ideal—cardinals prefer calm water, not strong currents. Flow should be 2-4 times your tank volume per hour (slower is fine in planted tanks). A tight-fitting lid prevents accidental jumps and reduces evaporation. Test kit for ammonia, nitrite, and nitrate is essential for monitoring water quality.

Maintenance: Perform 25-30% water changes weekly in moderate-stocking situations, or 15-20% every 2 weeks in lightly stocked planted tanks. Always match the temperature of replacement water to your tank water (within 1-2°F) to prevent thermal shock. Test ammonia and nitrite weekly—they must stay at 0 ppm. Keep nitrates below 30-40 ppm. Cardinals are sensitive to nitrogen compounds, especially in new or poorly maintained tanks.

Behavior

Cardinals are active, peaceful fish that explore your tank constantly during daylight hours—especially during early morning and late afternoon when they’d naturally be feeding in their blackwater streams.

What to Expect: In established tanks with good-sized schools (10+), they’re confident and bold. They’ll investigate new decorations, feed from the surface, and display beautiful dusk behavior where they dart around interactively as light fades. In smaller groups (6 or fewer), they tend to be shy and spend more time hiding. This isn’t a sign something’s wrong—it’s just how the species is wired. A 10-fish school is worlds different from a 6-fish group in terms of visible confidence.

Personality: Cardinals are curious once comfortable, but they’re not aggressive. They don’t fin-nip other fish, and they rarely show aggression toward each other. Mild chasing among males to establish dominance is normal and harmless. They’re skittish when stressed (faded colors, sudden movements, or aggressive tank mates will send them hiding), so stable conditions and proper tank design matter for keeping them visible.

Activity Patterns: Most active during morning and evening twilight. During bright midday, they may retreat to plants. At night, they rest among plants or on substrate. They’re crepuscular, meaning their activity peaks at dawn and dusk—this is when you’ll see their most impressive schooling behavior if you’re watching the tank during those times.

Tank Mates

Cardinals are peaceful community fish perfect for multi-species tanks. The key is choosing other small, peaceful species that prefer similar warm, soft, slightly acidic water.

Good Choices:

• Other tetras: Neon tetras, Black Neons, Green Neons, Rummy Nose, Ember tetras, or Glowlight tetras. They share identical water preferences and create stunning, colorful displays with multiple tetra species schooling together.
• Small rasboras: Harlequin rasboras, Chili rasboras, or Lambchop rasboras. Compatible in size, temperament, and water chemistry.
• Corydoras catfish: Bronze, Peppered, Panda, Pygmy, or Habrosus corydoras. They occupy the bottom (cardinals are midwater), have peaceful temperament, and help keep the substrate clean.
• Small loaches: Kuhli loaches or Dwarf chain loaches work well—they’re peaceful bottom-dwellers that prefer similar conditions.
• Small plecos: Bristlenose plecos or Otocinclus catfish help control algae and occupy different tank zones.
• Peaceful gouramis: Honey gouramis or Sparkling gouramis add color variety in larger tanks (20+ gallons).
• Freshwater shrimp: Cherry shrimp or Amano shrimp can work in heavily planted tanks, though cardinals may eat shrimplets.

Be Careful With:

• Male bettas: Can work in heavily planted 20+ gallon tanks, but individual betta temperament varies dramatically. Some coexist peacefully; others chase or attack cardinals. Success depends on the specific betta.
• Larger tetras: Congo tetras or Buenos Aires tetras may view small cardinals as competition or even food.
• Small barbs: Cherry barbs can work but may outcompete cardinals for food due to faster feeding behavior.

Avoid Completely:

• African cichlids: Completely incompatible—different water chemistry needs and too aggressive.
• Large South American cichlids: Oscars, Jack Dempseys, or Green Terrors will eat cardinals.
• Adult angelfish: While young angelfish work fine, mature angelfish can reach 6+ inches and may prey on cardinals.
• Tiger barbs: Notorious fin-nippers that harass peaceful fish.
• Pufferfish: Aggressive fin-nippers that specifically target small, colorful fish like cardinals.
• Goldfish: Completely different temperature requirements (cool water vs. cardinal’s warm water need) and they eat small fish.

Group Size: Keep cardinals in schools of at least 6, but 10-15+ is where their true personality emerges. Larger schools mean more confident, visible, interactive fish. It’s worth the investment.

Common Health Issues

Cardinals are generally hardy, but they’re sensitive to water quality issues and a few species-specific problems. Prevention through quarantine and excellent husbandry is far easier than treatment.

Ich (White Spot Disease): You’ll see tiny white spots resembling salt grains on their body and fins. They’ll also rub against decorations and plants (called “flashing”). Ich is caused by a parasite and is highly contagious but very treatable. Prevent it by quarantining all new fish for 2-4 weeks before adding them to your main tank. Maintain stable temperatures (76-80°F)—ich proliferates faster in cool water. Treat with ich medication according to package directions, raise temperature to 80-82°F to speed the parasite lifecycle, and perform 25% daily water changes. Treatment takes 7-14 days minimum.

Fin Rot: Frayed, deteriorating fin edges with white, red, or black discoloration. Fin rot is a bacterial infection, usually a secondary problem caused by stress or poor water quality. Prevention is straightforward: maintain pristine water quality with regular water changes, avoid overcrowding, remove sharp decorations, and quarantine new arrivals. Treat with antibiotics like Erythromycin or Furan-2 per package instructions, perform a 50% water change, and address the root cause (poor water quality or stress).

Neon Tetra Disease / Cardinal Tetra Disease: This is the scary one. It’s a parasitic or mycobacterial wasting disease that causes progressive color fading, a sunken, emaciated appearance, and lumps on the body. Unfortunately, there is currently no cure. Fish with suspected Neon Tetra Disease should be humanely euthanized to prevent suffering and disease spread. Prevention through strict quarantine (3-4 weeks), excellent water quality, and purchasing from reputable sources is the only reliable strategy.

New Fish: Always quarantine new cardinal tetras for 3-4 weeks in a separate tank before introducing them to your main tank. Watch for white spots, fin damage, fading color, wasting, abnormal swimming, lethargy, or lumps. Feed high-quality varied diet during quarantine to boost their immune system.

Lighting & Appearance

Light spectrum, intensity, and photoperiod directly affect how your cardinals look and behave.

Light Spectrum: Warm white light (3000K-4000K) emphasizes their red stripe and creates a warm appearance. Cool white light (6500K-7000K) makes the blue stripe glow electric and vibrant. For the best balance, use full-spectrum light (5000K-6500K) that brings out both colors naturally. Many aquarists prefer cool white or daylight spectrum for the stunning blue, but warm white is also beautiful—experiment and see what appeals to you.

Light Intensity: Low to moderate intensity (10-30 lumens per liter) is ideal. Bright lighting causes stress, fading, and constant hiding. If your current light is too bright, add floating plants to diffuse the light, or invest in dimmable LEDs. You’ll be surprised how much better cardinals look under moderate lighting—their colors appear more saturated because they’re exploring confidently instead of hiding.

Photoperiod: Aim for 8-10 hours of light per day. This replicates their natural low-light environment and minimizes algae growth. Extended photoperiods (12+ hours) cause chronic stress and excessive algae. Use an automatic timer to maintain consistency—cardinals appreciate routine.

Dark Backgrounds and Substrate: This is crucial. A dark (black or deep blue) background combined with dark substrate makes their stripes pop dramatically. It’s the difference between a pale fish and a glowing neon fish. The contrast between their colors and the background directly affects how vibrant they appear.

Breeding

Cardinal tetras are beautiful in community tanks, but breeding them is a specialized project—difficult enough that it’s considered an intermediate to advanced endeavor.

Why It’s Hard: Cardinals require extremely soft, acidic water (pH 4.0-5.5, GH 0-3 dGH—ideally 0-1 dGH). This means using 100% reverse osmosis (RO) water or rainwater; regular aquarium water is too hard and alkaline. Eggs are light-sensitive and must develop in complete darkness. Parents consume their own eggs and fry, so you need mechanical barriers (mesh or spawning mops) to protect them. Even with perfect conditions, spawning isn’t guaranteed—many conditioning cycles may be necessary.

If You Want to Attempt It: Set up a 5-10 gallon breeding tank with bare bottom, sponge filter, and RO water brought to pH 4.0-6.5 and GH 0-3. Condition males and females separately for 1-2 weeks on live foods (baby brine shrimp, daphnia). Perform 30-50% water changes with cooler water to trigger spawning (usually happens at night). Keep the tank in complete darkness. Once eggs hatch (24-36 hours), fry need infusoria and rotifers for the first week before accepting baby brine shrimp. Fry develop the characteristic blue and red stripes at 4-6 weeks. It’s a rewarding but labor-intensive process.

For Most Aquarists: Enjoy them in your community tank and leave breeding to the professionals. Captive-bred cardinals are readily available and affordable—there’s no shame in skipping the breeding project.

FAQ

Q: What’s the difference between cardinal tetras and neon tetras?
A: The main difference is the red stripe. Cardinal tetras have red stripes that run nearly the full length of their body (from behind the head to the tail). Neon tetras have red only on the back half. Cardinals are also slightly larger, hardier, and more robust than neons. Both are stunning, but cardinals are the beefier, tougher cousin.

Q: Do cardinals need to be in groups?
A: Absolutely. They’re schooling fish and should be kept in groups of at least 6, though 10-15+ is where you really see their personality. Small groups hide frequently and seem stressed. Large groups are confident, visible, and interactive—it’s a world of difference.

Q: Can I keep cardinals in a planted tank?
A: Yes, cardinals thrive in planted tanks. Dense planting with open swimming areas is ideal for them. Plants provide the shade they prefer and reduce stress while still allowing natural schooling behavior.

Q: How long do cardinal tetras live?
A: In well-maintained aquariums, 4-5 years is typical, though some live up to 8 years with excellent care. Lifespan depends heavily on water quality, temperature stability, and lack of stress.

Q: Why are my cardinals hiding and their colors faded?
A: Faded colors and hiding are stress indicators. Check water parameters (ammonia and nitrite must be 0), verify temperature stability (76-80°F), and assess tank mates (aggressive fish stress cardinals). Make sure your group is large enough (10+). Address the stressor and colors will return within days.

Q: Are cardinal tetras good for beginners?
A: They’re beginner-to-intermediate. They’re peaceful and beautiful, but they need established, cycled tanks and stable water parameters. If you already have a running aquarium and understand basic water chemistry, cardinals are a great choice. If you’re setting up your first tank, start with hardier fish like platies or danios first.

Conclusion

Cardinal tetras are one of the aquarium hobby’s true treasures. Their electric blue and crimson coloration, combined with their peaceful nature and schooling behavior, make them a joy to watch. They’re forgiving enough for beginners willing to learn about water chemistry, but rewarding enough to captivate experienced aquarists. Set them up right—warm, soft, slightly acidic water; a large school; dense plants; and dark substrate—and they’ll reward you with years of vibrant color and natural behavior. There’s something magical about watching a school of twenty cardinals catch the light during dusk, their stripes glowing like living neon. That’s why they’re one of the most beloved fish in the hobby, and once you keep them, you’ll understand why.

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Related Guides

• Neon Tetra Care Guide: The cardinal’s cousin—smaller, shorter red stripe, and slightly more delicate. Great for comparing the two species.
• Black Neon Tetra Care Guide: Another excellent tetra option with similar water parameter needs. Schools beautifully with cardinals.
• Planted Tank Setup for Beginners: Step-by-step guide for creating the planted tank cardinals love, with plant recommendations and lighting tips.
• Corydoras Catfish Care Guide: The perfect bottom-dwelling companion for cardinals in community tanks. Peaceful, helpful, and compatible with all parameters.