How many Cardinal Tetra in a 15 gallon tank?

A 15-gallon (57 L) tank comfortably holds about 6–9 Cardinal Tetra as a single-species display — a conservative range that leaves room for tankmates and a margin for error.

Cardinal Tetra key facts

TankStocking sizes this from a per-species footprint — the territory and waste a single Cardinal Tetra needs in a comfortable display tank — rather than the unreliable "inch per gallon" rule. On that basis a 15-gallon tank comfortably suits about 6–9 Cardinal Tetra. Keep a minimum of 6 — Cardinal Tetra is a social species that suffers in smaller groups. This is a comfortable display range, not a hard ceiling: stocking lighter keeps water quality stable and leaves headroom for tankmates and growth.

TankStocking's bio-load gauge alone would not flag a tank this size as over-stocked until roughly 25 Cardinal Tetra — but that is a model maximum, not the recommended number. It leaves no headroom for water-quality swings, tankmates, or growth, so treat 6–9 as the comfortable target.

They are gentle and easy to stock around, with the same one caveat as neons: keep them away from anything big enough to eat them. The engine clears small, peaceful community fish and flags larger predators. A big school in a planted tank is one of the hobby's great sights.

Want a community rather than a single-species tank? The Cardinal Tetra tank mates guide lists the species the engine clears with Cardinal Tetra.

Keeping Cardinal Tetra well: the detail

Beyond the table — the behaviour, the parameters that actually move the needle, and the mistakes that cost fish. Hobbyist context, not a verdict; the planner above is what judges your plan.

Water parameters in practice

Cardinals want warm, soft, acidic water — 25–28 °C and soft (under ~6 dGH), warmer and softer than neons, with which they are often muddled. They reward a mature, tannin-stained, planted tank and resent fresh, hard, alkaline water. Buy tank-bred stock if you can: wild-caught cardinals are beautiful but sensitive, while captive-bred are far hardier.

Why Cardinal Tetra behaves the way it does

Cardinal tetras come from the warm blackwater tributaries of the upper Rio Negro, mineral-poor and tea-coloured with leaf tannins, where they shoal over submerged litter in dim light. The full-length red stripe and the love of warmth and softness are direct readouts of that habitat. Recreate the dim, soft, warm conditions and the colour is extraordinary.

Common Cardinal Tetra stocking mistakes

The first mistake is treating a cardinal like a neon and keeping it too cool — they want the warm end. The second is adding wild-caught fish to an immature tank, where they fade and fail. The third is the same predation trap as neons: anything large-mouthed will eat a 3 cm cardinal.

Early signs something is wrong

A fading stripe, a fish swimming apart, and clamped fins are the early signs, usually pointing to cool or unstable water rather than disease. Wild stock is prone to internal parasites, so watch for thinness and stringy waste in new arrivals and quarantine them. A settled cardinal school glows and moves as one.

How tank size changes the answer

A school of six fits 15 gallons, but cardinals are made to be shown in numbers — a dozen or more in a 20-gallon-plus planted tank is one of the hobby's best sights. More water also holds the warmth and softness steadier, which this fish appreciates. Stock the group large and the tank dim.

Stocking in stages: cycle first, then add slowly

The stocking numbers on this page assume one thing the calculator can't do for you: a cycled tank, filled gradually. In a smaller tank the water chemistry swings faster, so adding everything at once is the quickest way to lose fish — stage it even more carefully than you would in a big tank.

1. Cycle before any fish. Run a fishless cycle — an ammonia source plus the filter — until the tank processes ammonia and nitrite to zero within 24 hours. That typically takes four to six weeks. This is the step that prevents most early losses.
2. Add one group at a time. Introduce a single species or group, then wait a fortnight before the next, testing the water in between. Each addition is a load the filter bacteria have to catch up with.
3. Heaviest waste producers last. Bring in the big, messy fish once the tank is mature and the bacteria are well established, not on day one.
4. Stop short of the limit. The comfortable numbers above leave headroom on purpose. Stocking lighter keeps water quality stable and gives you room to correct mistakes.

Test your real, staged plan in the planner above — it scores the full list against the same welfare engine, whatever order you intend to add fish in.

Common questions

More on stocking Cardinal Tetra

Related guides on TankStocking — each scored by the same welfare engine as the planner.

Stocking numbers are planning estimates — filtration, planting, and maintenance all shift the real ceiling. Cycle the tank and stock gradually. How this estimate works →