Compatibility Overview
Koi and Molly 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 | Molly | Verdict |
|---|---|---|---|
| Temperature | 50–75°F | 72–82°F | ⚠️ 72–75°F (narrow) |
| pH Range | 7.0–8.5 | 7.5–8.5 | ✅ 7.5–8.5 |
| Temperament | peaceful | peaceful | ✅ Compatible |
| Adult Size | 24 in | 4 in | ❌ 6× 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.
Molly — Poecilia sphenops
Hardy livebearers that prefer alkaline, hard water — sometimes slightly brackish. Prolific breeders; peaceful community fish.
Frequently Asked Questions
Can Koi live with Molly?
Generally not recommended. Koi and Molly 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 Molly together?
A minimum of 100 gallons is recommended for keeping Koi and Molly together. A larger tank improves water stability and reduces territorial conflict.
What temperature suits both Koi and Molly?
72–75°F (narrow)
Is a Koi aggressive toward a Molly?
Koi is peaceful and Molly is peaceful. Koi requires cold water; Molly requires tropical water. These environments are incompatible.