Compatibility Overview
Koi and Swordtail 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 | Koi | Swordtail | Verdict |
|---|---|---|---|
| Temperature | 50–75°F | 65–80°F | ✅ 65–75°F |
| pH Range | 7.0–8.5 | 7.0–8.3 | ✅ 7.0–8.3 |
| Temperament | peaceful | peaceful | ✅ Compatible |
| Adult Size | 24 in | 5 in | ❌ 5× difference |
| Min. Group | 2+ | 3+ | See below |
| Min. Tank Size | 100 gallons (for both species) | — | |
✅ Compatibility Positives
❌ Incompatibility Reasons
Species Profiles
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.
Swordtail — Xiphophorus hellerii
Hardy livebearer with a distinctive elongated lower tail fin in males. Peaceful community fish that tolerate a wide temperature range.
Frequently Asked Questions
Can Koi live with Swordtail?
Generally not recommended. Koi and Swordtail 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 Koi and Swordtail together?
A minimum of 100 gallons is recommended for keeping Koi and Swordtail together. A larger tank improves water stability and reduces territorial conflict.
What temperature suits both Koi and Swordtail?
65–75°F
Is a Koi aggressive toward a Swordtail?
Koi is peaceful and Swordtail is peaceful. Koi requires cold water; Swordtail requires tropical water. These environments are incompatible.