Photo: Wikimedia Commons contributor (Attribution) — via Wikimedia Commons
Cardinal Tetra (Paracheirodon axelrodi)
A living jewel from the Rio Negro: the full-length crimson stripe is the upgrade from the neon tetra that your blackwater biotope deserves.
Will it live with a Cardinal Tetra?
We compare each fish against your cardinal tetra on temperament, size, water parameters and swimming zone. Set your tank size and filter the results.
- African Dwarf Frog✅ CompatiblePeaceful · 4 cm · Medium care · 22–26 °C (72–79 °F)
- Compatible on the things that matter: shared water near 23–26 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Amapá Tetra✅ CompatiblePeaceful · 4 cm · Medium care · 24–27 °C (75–81 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Amapá Tetra in a shoal of 6+ or it gets stressed and nippy.
- Black Phantom Tetra✅ CompatiblePeaceful · 4.5 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Black Phantom Tetra in a shoal of 6+ or it gets stressed and nippy.
- Blue Danio✅ CompatiblePeaceful · 4 cm · Easy care · 21–26 °C (70–79 °F)
- Compatible on the things that matter: shared water near 23–26 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Blue Danio in a shoal of 6+ or it gets stressed and nippy.
- Blue Emperor Tetra✅ CompatiblePeaceful · 4 cm · Medium care · 23–27 °C (73–81 °F)
- Both are peaceful, and their water overlaps around 23–27 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Blue Emperor Tetra in a shoal of 6+ or it gets stressed and nippy.
- Emperor Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 23–27 °C (73–81 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Emperor Tetra in a shoal of 6+ or it gets stressed and nippy.
- Flame Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 22–28 °C (72–82 °F)
- Compatible on the things that matter: shared water near 23–27 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Flame Tetra in a shoal of 6+ or it gets stressed and nippy.
- Glowlight Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful, and their water overlaps around 23–27 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Glowlight Tetra in a shoal of 6+ or it gets stressed and nippy.
- Gold Tetra✅ CompatiblePeaceful · 4.5 cm · Easy care · 23–28 °C (73–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Gold Tetra in a shoal of 6+ or it gets stressed and nippy.
- Golden Dwarf Barb✅ CompatiblePeaceful · 4 cm · Easy care · 18–24 °C (64–75 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Golden Dwarf Barb in a shoal of 6+ or it gets stressed and nippy.
- Jelly Bean Tetra✅ CompatiblePeaceful · 4 cm · Medium care · 20–26 °C (68–79 °F)
- Compatible on the things that matter: shared water near 23–26 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Jelly Bean Tetra in a shoal of 10+ or it gets stressed and nippy.
- Lemon Tetra✅ CompatiblePeaceful · 4.5 cm · Easy care · 23–28 °C (73–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Lemon Tetra in a shoal of 6+ or it gets stressed and nippy.
- Marbled Hatchetfish✅ CompatiblePeaceful · 4 cm · Medium care · 23–28 °C (73–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Marbled Hatchetfish in a shoal of 6+ or it gets stressed and nippy.
- Otocinclus✅ CompatiblePeaceful · 4 cm · Medium care · 21–26 °C (70–79 °F)
- Both are peaceful, and their water overlaps around 23–26 °C — no size, zone or temperament conflicts.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Otocinclus in a shoal of 6+ or it gets stressed and nippy.
- Phoenix Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 23–28 °C (73–82 °F)
- Compatible on the things that matter: shared water near 23–27 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Phoenix Tetra in a shoal of 8+ or it gets stressed and nippy.
- Purple Tetra✅ CompatiblePeaceful · 4 cm · Medium care · 22–26 °C (72–79 °F)
- Both are peaceful, and their water overlaps around 23–26 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Purple Tetra in a shoal of 8+ or it gets stressed and nippy.
- Rainbow Emperor Tetra✅ CompatibleSemi-aggressive · 3.6 cm · Medium care · 22–28 °C (72–82 °F)
- Peaceful + Semi-aggressive, but with no direct clash here, and their water overlaps around 23–27 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Rainbow Emperor Tetra in a shoal of 8+ or it gets stressed and nippy.
- Red Phantom Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 22–28 °C (72–82 °F)
- Compatible on the things that matter: shared water near 23–27 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Red Phantom Tetra in a shoal of 8+ or it gets stressed and nippy.
- Rosy Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful, and their water overlaps around 23–27 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Rosy Tetra in a shoal of 6+ or it gets stressed and nippy.
- Sparkling Gourami✅ CompatiblePeaceful · 4 cm · Medium care · 24–28 °C (75–82 °F)
- Both are peaceful, and their water overlaps around 24–27 °C — no size, zone or temperament conflicts.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Strawberry Betta✅ CompatiblePeaceful · 4 cm · Medium care · 22–27 °C (72–81 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Threadfin Rainbowfish✅ CompatiblePeaceful · 4 cm · Medium care · 24–28 °C (75–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Tiger Otocinclus✅ CompatiblePeaceful · 4 cm · Medium care · 22–27 °C (72–81 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Tiger Otocinclus in a shoal of 6+ or it gets stressed and nippy.
- Yellow Phantom Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 23–27 °C (73–81 °F)
- Both are peaceful, and their water overlaps around 23–27 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Yellow Phantom Tetra in a shoal of 6+ or it gets stressed and nippy.
- Assassin Snail⚠️ With cautionPeaceful · 3 cm · Easy care · 22–26 °C (72–79 °F)
- Water hardness preferences differ (Cardinal Tetra 1–6 vs Assassin Snail 8–20 dGH).
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Black Darter Tetra⚠️ With cautionSemi-aggressive · 4 cm · Hard care · 21–28 °C (70–82 °F)
- Black Darter Tetra and Cardinal Tetra are close in size, but the semi-aggressive one tends to dominate — add cardinal tetra in a group to spread the pressure.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Blue Turbo Snail⚠️ With cautionPeaceful · 5 cm · Medium care · 25–30 °C (77–86 °F)
- Different pH ranges (4.5–7 vs 7.5–8.5); doable if you sit in the shared band, but not ideal long-term.
- Water hardness preferences differ (Cardinal Tetra 1–6 vs Blue Turbo Snail 8–18 dGH).
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Cherry Shrimp⚠️ With cautionPeaceful · 3 cm · Easy care · 18–28 °C (64–82 °F)
- Adult Cherry Shrimp might survive with Cardinal Tetra, but expect the young to be eaten — plant heavily.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Cherry Shrimp in a shoal of 10+ or it gets stressed and nippy.
- Endler's Livebearer⚠️ With cautionPeaceful · 3 cm · Easy care · 22–28 °C (72–82 °F)
- Water hardness preferences differ (Cardinal Tetra 1–6 vs Endler's Livebearer 10–25 dGH).
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Endler's Livebearer in a shoal of 6+ or it gets stressed and nippy.
- Ghost Shrimp⚠️ With cautionPeaceful · 4 cm · Easy care · 18–28 °C (64–82 °F)
- Adult Ghost Shrimp might survive with Cardinal Tetra, but expect the young to be eaten — plant heavily.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Ghost Shrimp in a shoal of 6+ or it gets stressed and nippy.
- Humpbacked Tetra⚠️ With cautionSemi-aggressive · 5 cm · Hard care · 24–28 °C (75–82 °F)
- Humpbacked Tetra is semi-aggressive and may chase or nip the smaller Cardinal Tetra — plant heavily and break up sight lines.
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Humpbacked Tetra in a shoal of 8+ or it gets stressed and nippy.
- Morse Code Corydoras⚠️ With cautionPeaceful · 5 cm · Medium care · 23–26 °C (73–79 °F)
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Morse Code Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Mystery Snail⚠️ With cautionPeaceful · 5 cm · Easy care · 20–28 °C (68–82 °F)
- Water hardness preferences differ (Cardinal Tetra 1–6 vs Mystery Snail 7–18 dGH).
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Rummy Nose Rasbora⚠️ With cautionPeaceful · 4 cm · Medium care · 20–26 °C (68–79 °F)
- Water hardness preferences differ (Cardinal Tetra 1–6 vs Rummy Nose Rasbora 8–16 dGH).
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Rummy Nose Rasbora in a shoal of 8+ or it gets stressed and nippy.
- Serpae Tetra⚠️ With cautionSemi-aggressive · 4 cm · Easy care · 23–28 °C (73–82 °F)
- Serpae Tetra is semi-aggressive and may chase or nip the smaller Cardinal Tetra — plant heavily and break up sight lines.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Serpae Tetra in a shoal of 6+ or it gets stressed and nippy.
- Silvertip Tetra⚠️ With cautionSemi-aggressive · 5 cm · Easy care · 22–28 °C (72–82 °F)
- Silvertip Tetra is semi-aggressive and may chase or nip the smaller Cardinal Tetra — plant heavily and break up sight lines.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Keep Silvertip Tetra in a shoal of 6+ or it gets stressed and nippy.
- Spotfin Betta⚠️ With cautionSemi-aggressive · 5 cm · Medium care · 23–28 °C (73–82 °F)
- Spotfin Betta and Cardinal Tetra are close in size, but the semi-aggressive one tends to dominate — add cardinal tetra in a group to spread the pressure.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Striped Red-Eye Puffer⚠️ With cautionAggressive · 5 cm · Hard care · 24–28 °C (75–82 °F)
- Striped Red-Eye Puffer is aggressive and may chase or nip the smaller Cardinal Tetra — plant heavily and break up sight lines.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Tiger Badis⚠️ With cautionSemi-aggressive · 4 cm · Medium care · 22–24 °C (72–75 °F)
- Tiger Badis is semi-aggressive and may chase or nip the smaller Cardinal Tetra — plant heavily and break up sight lines.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Wine Red Betta⚠️ With cautionAggressive · 5 cm · Medium care · 24–28 °C (75–82 °F)
- Expect Wine Red Betta to harass Cardinal Tetra at times; give dense cover and watch them at feeding.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Alligator Gar⛔ Not recommendedAggressive · 250 cm · Hard care · 24–28 °C (75–82 °F)
- Cardinal Tetra is bite-sized to a 250 cm predatory alligator gar — it will be eaten.
- Alligator Gar clearly outsizes Cardinal Tetra and is aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~3785 L this pairing really wants — crowding raises aggression.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Clown Knifefish⛔ Not recommendedAggressive · 90 cm · Hard care · 24–28 °C (75–82 °F)
- Clown Knifefish (90 cm) is big enough to swallow the 4 cm Cardinal Tetra whole.
- Clown Knifefish is aggressive and may chase or nip the smaller Cardinal Tetra — plant heavily and break up sight lines.
- Your 75 L tank is below the ~750 L this pairing really wants — crowding raises aggression.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Fire Eel⛔ Not recommendedSemi-aggressive · 100 cm · Medium care · 24–28 °C (75–82 °F)
- Fire Eel (100 cm) is big enough to swallow the 4 cm Cardinal Tetra whole.
- Expect Fire Eel to harass Cardinal Tetra at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~380 L this pairing really wants — crowding raises aggression.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Koi⛔ Not recommendedPeaceful · 90 cm · Medium care · 4–28 °C (39–82 °F)
- Koi (90 cm) is big enough to swallow the 4 cm Cardinal Tetra whole.
- Water hardness preferences differ (Cardinal Tetra 1–6 vs Koi 9–18 dGH).
- Your 75 L tank is below the ~3800 L this pairing really wants — crowding raises aggression.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Redtail Catfish⛔ Not recommendedAggressive · 120 cm · Hard care · 24–27 °C (75–81 °F)
- Cardinal Tetra is bite-sized to a 120 cm predatory redtail catfish — it will be eaten.
- Redtail Catfish is aggressive and may chase or nip the smaller Cardinal Tetra — plant heavily and break up sight lines.
- Your 75 L tank is below the ~5700 L this pairing really wants — crowding raises aggression.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Spotted Gar⛔ Not recommendedAggressive · 90 cm · Hard care · 18–26 °C (64–79 °F)
- Size gap is too large (90 vs 4 cm): Spotted Gar will treat Cardinal Tetra as food.
- Spotted Gar clearly outsizes Cardinal Tetra and is aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~600 L this pairing really wants — crowding raises aggression.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Wels Catfish⛔ Not recommendedAggressive · 300 cm · Hard care · 15–25 °C (59–77 °F)
- Size gap is too large (300 vs 4 cm): Wels Catfish will treat Cardinal Tetra as food.
- Wels Catfish is aggressive and may chase or nip the smaller Cardinal Tetra — plant heavily and break up sight lines.
- Your 75 L tank is below the ~20000 L this pairing really wants — crowding raises aggression.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Wolf Cichlid⛔ Not recommendedAggressive · 72 cm · Hard care · 24–30 °C (75–86 °F)
- Cardinal Tetra is bite-sized to a 72 cm predatory wolf cichlid — it will be eaten.
- Water hardness preferences differ (Cardinal Tetra 1–6 vs Wolf Cichlid 8–20 dGH).
- Wolf Cichlid clearly outsizes Cardinal Tetra and is aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~760 L this pairing really wants — crowding raises aggression.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
Compatibility is computed from each species' care data — a strong starting point, not a guarantee. Individual temperament varies, so always introduce new fish slowly and watch them.
Cardinal Tetra care specs
- Care level
- Medium
- Breeding
- Very Hard
- Max size
- 4 cm (1.6 in)
- Min tank size
- 60 L (15.9 gal)
- Temperature
- 23–27 °C (73–81 °F)
- pH
- 4.5–7
- Hardness
- 1–6 dGH
- Lifespan
- 3–5 years
- Diet
- Omnivore
- Swim level
- Middle
- Group size
- 6+ (shoaling)
- Family
- Characidae
- Origin
- South America — Rio Negro and Orinoco basins (Colombia, Venezuela, Brazil)
What is a Cardinal Tetra?
The cardinal tetra (Paracheirodon axelrodi) is a small freshwater characid widely regarded as one of the most beautiful shoaling fish available to the hobby. Reaching just 4 cm (1.6 in), it punches far above its size: a dense shoal in a well-planted blackwater aquarium produces a continuous ribbon of electric blue-green and full-length crimson that few fish can rival.
Unlike its close relative the neon tetra (Paracheirodon innesi), the cardinal’s red stripe runs the entire length of the belly from the chin to the base of the tail — that full stripe is the quickest identification mark and what makes it visually superior in a large group. Cardinals are fully peaceful mid-water shoalers, compatible with any calm fish that shares their water-chemistry requirements.
Where do Cardinal Tetras come from?
Cardinal tetras are native to the Rio Negro and Orinoco basins of South America — the rivers and flooded forests of Colombia, Venezuela, and Brazil. These waterways are classic blackwater environments: the water is stained dark amber by decaying leaf litter and wood, is almost devoid of dissolved minerals, and is soft and acidic in a way that most tropical tap water simply cannot replicate without effort.
Wild cardinals school in enormous numbers in dimly lit, slow-moving forest streams. Because this habitat is so distinctive, the species is highly sensitive to mineral content and pH outside its native range. Almost all cardinal tetras sold in the aquarium trade are wild-caught under Brazil’s sustainable collection programme rather than captive-bred — a factor worth keeping in mind when you buy them, as wild-caught fish may need a careful, patient acclimatisation to your tank water.
What size tank does a Cardinal Tetra need?
The minimum practical tank size is 60 litres (16 gallons), and that is for a modest group. A longer footprint — think 80–120 cm (31–47 in) — is better than a tall tank, because cardinals are mid-water swimmers that use horizontal space for shoaling behaviour. In a 90 L (24 gal) or larger tank you can keep a more impressive group of 12–15, which is where their schooling display becomes genuinely spectacular.
A tightly fitting lid is useful, as cardinals can jump when startled. Heavy planting, floating plants to diffuse light, dark substrate (black sand or fine gravel), and a few pieces of driftwood or Indian almond leaves transform a plain aquarium into the kind of environment where cardinals thrive and show their colours at full intensity.
What water parameters do Cardinal Tetras need?
Cardinals are the archetypal soft-water, acidic-water fish. Their native Rio Negro water is among the softest and most acidic in the world.
- Temperature: 23–27 °C (73–81 °F). Keep the range stable; swings weaken their immune response.
- pH: 4.5–7.0. Practically, 5.5–6.8 suits most fish in the tank while still keeping cardinals comfortable. Lower pH is fine if the whole setup is oriented that way.
- Hardness: 1–6 dGH. This is genuinely soft water — typical UK or US moderately hard tap water at 10–15 dGH is too hard for long-term health without treatment.
To hit these numbers reliably, many keepers blend reverse-osmosis (RO) water with a small amount of mineralised tap water, or use rainwater, then soften and acidify with peat filtration or Indian almond leaves. Ammonia and nitrite must be zero; cardinals are sensitive to any cycle instability and should only ever be added to a fully established, mature tank.
What do Cardinal Tetras eat?
Cardinal tetras are omnivores with a natural diet of tiny invertebrates, zooplankton, and some plant matter from the forest-floor substrate. In the aquarium they accept a wide range of prepared and live foods.
A good feeding routine:
- Staple: high-quality micro-pellets or fine crushed flake appropriate for small characids — look for products with a high protein content.
- Variety: frozen or live daphnia, baby brine shrimp, micro-worms, and bloodworms (finely chopped or small-grade frozen). These are excellent for conditioning and colour.
- Feeding frequency: twice daily in small amounts. Cardinals have tiny mouths — over-sized food is wasted and rots quickly in soft, low-pH water.
Because their water is naturally low in buffering capacity, decaying uneaten food degrades water quality especially fast. Feed only what the group consumes within two minutes.
Are Cardinal Tetras aggressive — and what fish can live with them?
Cardinal tetras are completely peaceful and are one of the most community-friendly fish available. They pose no threat to any tank mate; their only real vulnerability is to larger fish that might eat them. Their small adult size of 4 cm (1.6 in) means anything with a mouth wider than about 3 cm is a potential predator.
The critical compatibility filter is water chemistry, not temperament. Cardinals require soft, acidic water that is incompatible with livebearers, African cichlids, goldfish, or other hard-water species. Within a shared soft-water community, excellent companions include:
- Dwarf cichlids (Apistogramma spp., Mikrogeophagus ramirezi) — share the same parameter window and occupy the lower water column, avoiding competition.
- Corydoras — peaceful bottom-dwellers that tolerate soft, slightly acidic water.
- Hatchetfish (Carnegiella or Gasteropelecus spp.) — surface dwellers that complement cardinals in the mid-column.
- Other small soft-water tetras and rasboras that share the chemistry requirements.
Keep cardinals in a group of at least 6, and ideally 10 or more. Below that number the shoal becomes timid, fish hide, and the visual effect is lost. For a detailed, filterable list of compatible species, see Cardinal Tetra tank mates.
How do you tell male and female Cardinal Tetras apart?
Sexual dimorphism in cardinal tetras is subtle and not reliably visible outside of the breeding season. The most useful cue is body shape: females become noticeably rounder in the belly when gravid (carrying eggs), and the iridescent blue stripe along the back may appear to bow or curve slightly around the fuller abdomen rather than running perfectly straight. Males tend to be slimmer and more streamlined.
Unless you have a conditioned group in spawning condition side by side, distinguishing males from females in a typical shop group or community tank is genuinely difficult. For most aquarists this distinction is academic, as captive breeding at the hobbyist level is rarely attempted.
How do Cardinal Tetras breed?
Breeding cardinal tetras in captivity is rated very hard and is not a realistic goal for most fishkeepers. The conditions required are extreme even by soft-water standards.
Wild cardinals spawn in near-darkness in water with a pH as low as 4.0–5.0 and hardness approaching zero. The eggs are photosensitive and will fail to develop in any meaningful light. To attempt a spawn, experienced breeders set up a dedicated breeding tank using RO water, peat filtration to achieve the required pH and zero hardness, and full blackout conditions. Even then, egg viability is often low and raising the fry on infusoria and micro-foods requires significant effort.
For the vast majority of the hobby, cardinal tetras are a buy, don’t breed species. The sustainable wild-collection programme in Brazil is well-regulated, and wild-caught fish are healthy and available year-round.
What are common Cardinal Tetra diseases?
Cardinals’ soft-water, acidic environment naturally suppresses many bacterial and fungal pathogens, which is one reason a well-maintained blackwater tank often sees fewer disease outbreaks than a hard-water community. That said, the usual freshwater concerns can still occur:
- Ich (white spot): Small white pinhead dots across body and fins. Almost always triggered by a temperature drop or stress from poor water quality or rough transport.
- Fin rot: Ragged or receding fin edges, typically bacterial in origin and a direct signal of declining water quality or a compromised immune system.
- Neon tetra disease (Pleistophora hyphessobryconis): A microsporidian parasite that also affects cardinals. Look for faded or irregular colour patches, curved spine, difficulty swimming. There is no reliable cure; affected fish should be removed and isolated promptly to prevent spread.
- Velvet (Oodinium): A fine gold or rust-coloured dust visible in raking light. Stress and poor water quality are the usual triggers.
Prevention follows a consistent theme: maintain pristine water quality, perform regular partial water changes, quarantine all new fish for 2–4 weeks before adding them to the main tank, and avoid sudden parameter shifts. The single biggest risk factor for cardinals is adding them to an uncycled or recently destabilised tank.
Health note: confirming a specific disease before treating is important — many conditions share overlapping symptoms. Consult a reputable veterinary or specialist aquarium-health source before reaching for any medication, and always check whether a proposed treatment is safe for the low pH and soft water of a cardinal tetra setup.
How long do Cardinal Tetras live?
In good conditions, cardinal tetras live 3–5 years. That lifespan assumes stable, species-appropriate water chemistry, a nutritious varied diet, and a low-stress environment with a proper shoal of at least six fish.
Because so much retail stock is wild-caught and may have experienced the stress of long-distance transport, newly purchased cardinals can be fragile in the first few weeks. A well-managed quarantine period and a slow, careful acclimatisation to your tank water give them the best chance of settling in and reaching their full lifespan. Once established in a mature soft-water setup, they are hardy, rewarding long-term residents.
Frequently asked questions
What is the difference between a cardinal tetra and a neon tetra?
The red stripe tells you immediately: on a neon it runs only from the middle of the body to the tail, while on a cardinal it extends the full length of the belly from chin to tail. Cardinals also grow slightly larger and demand softer, more acidic water. They are less forgiving of water quality than neons, which is why neons are rated Easy and cardinals Medium.
Why are cardinal tetras so hard to breed in captivity?
They spawn in very soft, extremely acidic blackwater (pH as low as 4.0–5.0) and the eggs are highly sensitive to light and any mineral content in the water. Even experienced breeders use RO water, peat filtration and blackout conditions to get viable spawns. For most hobbyists they are a buy-don't-breed species — almost all stock sold in shops is wild-caught from sustainable collection programs in Brazil.
What you need to keep a cardinal tetra
The baseline is a heated, filtered 60 L+ tank: a reliable heater to hold 23–27 °C (73–81 °F), a gentle filter that won't batter a cardinal tetra in the current, and a tight-fitting lid. Cycle the tank fully before adding any fish.
As an Amazon Associate we earn from qualifying purchases — buying through these links costs you nothing extra.
