Cardinal Tetra
Fish
The neon tetra's flashier cousin, lit up red from nose to tail!
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🤓 Did you know? Huge numbers of cardinal tetras are still sustainably wild-harvested from Brazil's Rio Negro every year through community-based fisheries, a rare case where the aquarium trade directly funds rainforest conservation by giving local communities a real economic reason to keep the river intact.
🗓️ The first week
The numbers a new owner needs in the first week, each from the article it links to. Rows with no link come from the encyclopedia.
- Before any fish
- A 2008 study on this species found it tolerates ammonia better than assumed but is sensitive to nitrite, so a finished cycle before stocking is the single most useful habit.Health guide →
- School size
- Ten or more. Smaller groups develop stress sickness, and cardinals shoal tighter and color up better in bigger groups. Six is a floor, not a target.Handling guide →
- Tank size
- A 20-gallon long, roughly 24x12 inches of base, for the 8 to 10 fish school. A 10-gallon holds only the bare minimum of 6.Setup guide →
- Temperature
- 77 to 82°F for general care, inside a tolerated 73 to 84°F. Cardinals take water up to 86°F, against roughly 77°F for neons, so they suit a warmer community tank.Setup guide →
- Water chemistry
- Soft and acidic, pH 5.0 to 7.0 and toward the low end. Test your source water first rather than assuming it fits, and ammonia and nitrite read zero before stocking.Setup guide →
- Filtration and substrate
- Gentle flow, a sponge filter is the common choice, over fine gravel or sand. A darker substrate makes the red and blue stripe stand out.Setup guide →
- Light and blackwater
- Dim to moderate light, driftwood and Indian almond leaves for tannins, dense planting. Bright light over bare glass leaves them pale and near the bottom.Setup guide →
- Mostly wild caught
- Still largely collected from the Rio Negro, and losses in the first fortnight are common. Buy from a shop that has held them a while, acclimate slowly, dim the lights and leave them alone.Enrichment guide →
- Diet
- A micro-pellet or crushed flake as the staple, with live or frozen bloodworms, mosquito larvae or daphnia. Whole flakes and standard pellets are too big for the mouth and get spat out, which starves the fish.Feeding guide →
- Feeding schedule
- Twice a day, what the school finishes in a couple of minutes, or split into three or four smaller feeds. Food still visible after that means cut back: nitrite is the parameter they tolerate worst.Feeding guide →
- Water changes
- 25 to 30% weekly from the substrate, with conditioned water at tank temperature. Test nitrate first: under 20 ppm the routine is right, past 40 ppm the next change is 50%.Aquarium Water Changes →
- A dead fish
- Never leave one in the tank or let the others scavenge it. There is no cure for neon tetra disease, and fish that eat an infected body contract it.Feeding guide →
- Quarantine
- A full 30 days for any new fish, the most effective way to keep ich out of an established tank.Health guide →
- Budget
- $1 to $3 a fish, roughly $8 to $30 for the school, and most first setups land around $100 to $250. Vet care does not apply; water quality and over-the-counter treatment are the whole of it.Cost guide →
- Lifespan
- About five years, some to ten. In the wild it is close to an annual fish.Cost guide →
- Adult size
- 2 inches (5 cm).
- Water chemistry, the wider picture
- KH holds pH in place, and dosing pH-up or pH-down against it is how keepers crash a tank. Work with what the source water gives you.Freshwater pH, GH, and KH →
- Power outage
- Oxygen is the threat, not darkness. Unplug the filter rather than letting it sit dead in the water, and keep the surface moving.Power outage guide →
🚨 Emergency card
Test the water first, then act on these today. Aquatic vets are rare, so nearly all of this is handled at home.
- Small white spots like grains of salt, flashing against decor, rapid breathing: ich, treated with medication and not with heat, since this species dies near 33.7°C
- Discoloration on the fin edges, then fraying: fin rot, treatable
- Floating at the surface unable to swim down, or struggling to rise: poor water quality first, then overfeeding
- Restless or erratic swimming at night, fading or patchy color, lumps under the skin, a curved spine: neon tetra disease, no cure, and the fish is separated so the others cannot eat it
Everything treatable here traces back to water quality and stability. The untreatable one is prevented by quarantine.
📰 Where to go next
How Much Does Cardinal Tetra Ownership Really Cost?
$1 to $3 a fish, the $65 to $230 setup, and the Rio Negro fishery behind why this one prices above a neon.
Cardinal Tetra Tank Setup: Soft Water, Warmer Temps, and a Blackwater Feel
The 20-gallon long, 73 to 84°F, soft acidic water, gentle flow, and the blackwater tannins that change the fish.
Cardinal Tetra Feeding: Schedule, Diet, and the Rule That Prevents an Outbreak
Micro-pellet portions, the two-minute rule, and why removing a dead fish immediately is the real defense against an incurable disease.
Cardinal Tetras Aren't Handled Either, But the School Size Rule Is Even Stricter
Why this is an observation fish, the ten-fish minimum that functions as a welfare need, and the stripe that tells it from a neon.
Common Cardinal Tetra Health Issues (Including a Disease Named After a Different Fish)
Neon tetra disease, ich, fin rot, swim bladder trouble, and the 2008 study that says nitrite, not ammonia, is the number to watch.
Cardinal Tetra Enrichment: Most of Them Are Still Wild
Most of them are still wild caught, what that means for the first fortnight, and the borrowed neon tetra finding that says ten.
🛒 What to buy
Prices and the reasoning behind each item are in How Much Does Cardinal Tetra Ownership Really Cost?.
❓ Frequently Asked Questions
How many cardinal tetras should I keep together?
Animal Diversity Web puts the workable minimum at 10, noting that fewer fish kept together tend to develop sickness from stress, and Seriously Fish recommends starting with 8 to 10 or more for natural schooling behavior, a notably higher bar than the six-fish floor commonly cited for neon tetra. A group that's too small produces fish that are pale, stressed, and constantly hiding.
Can cardinal tetras get neon tetra disease?
Yes. It's named after neon tetra specifically, but the Merck Veterinary Manual documents the parasite affecting tetras broadly, along with angelfish, rasboras, barbs, and zebrafish, so cardinal tetra falls within that group. It's caused by a microsporidian parasite, Pleistophora hyphessobryconis, that invades muscle tissue. Watch for restless or erratic swimming, fading or patchy color, visible lumps under the skin, and a curved spine in advanced cases.
What's the non-obvious detail that makes the biggest difference in a cardinal tetra tank?
Recreating blackwater conditions. Driftwood and Indian almond leaves release natural tannins that stain the water and lower pH, mimicking the shaded, leaf-litter-covered streams this species evolved in. Pair that with dense planting and dim lighting, and color and confidence both improve noticeably.
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📰 Deep Dive
🐠 How Much Does Cardinal Tetra Ownership Really Cost?
🐠 Cardinal Tetras Aren't Handled Either, But the School Size Rule Is Even Stricter
🐠 Common Cardinal Tetra Health Issues (Including a Disease Named After a Different Fish)
🐠 Cardinal Tetra Tank Setup: Soft Water, Warmer Temps, and a Blackwater Feel
🐠 Cardinal Tetra Feeding: Schedule, Diet, and the Rule That Prevents an Outbreak
🐠 Cardinal Tetra Enrichment: Most of Them Are Still Wild
🩺 Health and More
🐠 Aquarium Power Outages, Transport, and Moving Your Fish: The Complete Plan
🦠 How to Cycle a Fish Tank, Start to Finish: Fishless and Fish-In
🧽 Aquarium Filtration: How It Actually Works and How to Maintain It
🪣 Aquarium Water Changes: How Much, How Often, and What the Number Is For
🐟 Aquarium Stocking and Tankmates: How Many Fish, and Which Ones Go Together
🧪 Freshwater pH, GH, and KH: Why Chasing a Number Backfires
🤩 Fun Facts
🐠 A Stripe That Hides You
That vivid red-and-blue stripe isn't just decoration. In the dim, tannin-stained water this species evolved in, the metallic stripe actually functions as camouflage, reflecting available light in a way that creates false or confusing targets for predators. What looks like the showiest feature in a home aquarium is doing real defensive work in a blackwater stream.