Cardinal Tetra Tank Setup: Soft Water, Warmer Temps, and a Blackwater Feel

Cardinal tetras are native to the upper Orinoco basin and the Negro River in the Amazon basin, according to a 2023 phylogeography study of the species, the same kind of soft, dim, tannin-stained blackwater country neon tetra comes from. This species just runs a few degrees warmer and is usually kept in a slightly bigger school.

Tank Size

A 10-gallon tank is workable for the bare minimum school of 6. Seriously Fish recommends a base footprint of roughly 60x30cm, close to 24x12 inches, which lines up well with a 20-gallon long tank rather than a standard 10-gallon. If you're stocking the fuller 8 to 10 fish school this species is usually kept in, that's the better practical minimum. Schooling behavior plays out horizontally, so favor a longer tank over a taller one.

A 10-gallon tank covers a school of 6 at minimum. For the more typical 8 to 10 fish school, a 20-gallon long tank is worth the upsize, our handling guide covers why the school size itself matters as much as the tank does.

Temperature

73 to 84°F is the range Seriously Fish gives for this species, while Practical Fishkeeping puts general care closer to 77 to 82°F and notes that cardinal tetra tolerates water reaching 30°C (86°F), warmer than the roughly 25°C (77°F) ceiling neon tetra is comfortable with. A heater is necessary for any tank this size, and keeping the temperature stable matters as much as hitting the right number.

Water Chemistry

Cardinal tetras want soft, acidic water. Seriously Fish gives a comfortable range of pH 3.5 to 7.5, noting that wild-caught specimens do best toward the more acidic end. In practice, aiming for roughly pH 5.0 to 7.0, toward the lower end where possible, works well, and it's worth testing your source water before assuming your setup is automatically suitable. Ammonia and nitrite both need to read zero before stocking.

Filtration

Keep the flow gentle. Seriously Fish describes this species' native habitat as slow-moving forest streams full of fallen branches and leaf litter, not open water with strong current. A sponge filter is a common, effective choice, and a hang-on-back filter works too if you can baffle or reduce its output.

Substrate

Fine gravel or sand, and a darker color makes cardinal tetra's red and blue stripe stand out more against the background, the same principle that applies to the closely related neon tetra.

Lighting

Dim to moderate. This species comes from shaded, densely vegetated streams, and bright, intense lighting stresses fish built for filtered, dappled light rather than open water.

The Non-Obvious Requirement: Recreate Blackwater Conditions

This is the detail that takes a cardinal tetra tank from adequate to genuinely great. Driftwood and Indian almond leaves release natural tannins that stain the water and lower pH, mimicking the blackwater habitat this species evolved in, the same fallen branches and leaf litter Seriously Fish describes as the defining feature of its native streams.

Pair this with dense midground live plants to dim the light further and give the school somewhere to retreat. Always quarantine new fish before adding them, given the disease risk this species carries, and a bare hospital tank is what that means, not a spare corner of the display tank. Keep them well fed on a proper micro-pellet food sized for their small mouths. Always use a water conditioner on tap water before it goes in the tank, dechlorinating and neutralizing chloramine and heavy metals that are otherwise harmful to fish.

The Cardinal tetra encyclopedia profile has more on the species itself.

Sources & Further Reading