Compatibility Overview
Cardinal Tetra and Koi are generally not recommended to be housed together. The issues listed above — whether water parameter incompatibility, predation risk, or aggression — create conditions where one or both fish are likely to suffer harm or chronic stress.
| Factor | Cardinal Tetra | Koi | Verdict |
|---|---|---|---|
| Temperature | 73–82°F | 50–75°F | ⚠️ 73–75°F (narrow) |
| pH Range | 4.5–7.0 | 7.0–8.5 | ⚠️ 7.0–7.0 (narrow) |
| Temperament | peaceful | peaceful | ✅ Compatible |
| Adult Size | 1.5 in | 24 in | ❌ 16× difference |
| Min. Group | 6+ | 2+ | See below |
| Min. Tank Size | 100 gallons (for both species) | — | |
✅ Compatibility Positives
❌ Incompatibility Reasons
Species Profiles
Cardinal Tetra — Paracheirodon axelrodi
Brilliant red-and-blue schooling fish from the Rio Negro. Prefers soft, acidic, warm water — a natural companion for discus.
→ Full Cardinal Tetra care guide
Koi — Cyprinus rubrofuscus
Large pond fish that require hundreds of gallons and cool water. Will eat any fish small enough to fit in their mouth.
Frequently Asked Questions
Can Cardinal Tetra live with Koi?
Generally not recommended. Cardinal Tetra and Koi are generally not recommended to be housed together. The issues listed above — whether water parameter incompatibility, predation risk, or aggression — create conditions where one or both fish are likely to suffer harm or chronic stress.
What tank size do I need for Cardinal Tetra and Koi together?
A minimum of 100 gallons is recommended for keeping Cardinal Tetra and Koi together. A larger tank improves water stability and reduces territorial conflict.
What temperature suits both Cardinal Tetra and Koi?
73–75°F (narrow)
Is a Cardinal Tetra aggressive toward a Koi?
Cardinal Tetra is peaceful and Koi is peaceful. Cardinal Tetra requires tropical water; Koi requires cold water. These environments are incompatible.