Koi and Molly Compatibility Overview
Koi and Molly are generally not recommended to be housed together. Koi is a cool-water fish and Molly is tropical, so the usable temperature range decides this pairing. The adult size gap creates a meaningful predation risk. Review the behavior and water details before deciding whether to attempt the pairing.
| Factor | Koi | Molly | Verdict |
|---|---|---|---|
| Temperature | 50โ75ยฐF | 72โ82ยฐF | โ ๏ธ 72โ75ยฐF (narrow) |
| pH Range | 7.0โ8.5 | 7.5โ8.5 | โ 7.5โ8.5 |
| Temperament | peaceful | peaceful | โ Compatible |
| Adult Size | 24 in | 4 in | โ 6ร difference |
| Min. Group | 2+ | 3+ | See below |
| Min. Tank Size | 100 gallons (for both species) | โ | |
Compatibility Positives
Incompatibility Reasons
Species Profiles
Koi โ 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 โ Poecilia sphenops
Hardy livebearers that prefer alkaline, hard water โ 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. Koi is a cool-water fish and Molly is tropical, so the usable temperature range decides this pairing. The adult size gap creates a meaningful predation risk. Review the behavior and water details before deciding whether to attempt the pairing.
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?
72โ75ยฐF (narrow) โ inside Koi's 50โ75ยฐF and Molly's 72โ82ยฐF.
Is a Koi aggressive toward a Molly?
The adult size gap creates a meaningful predation risk.
Related Compatibility Guides
Tank Size for Koi and Molly
Use the scenario that matches the actual group sizes and temperament you plan to keep.
| Scenario | Tank | When it applies |
|---|---|---|
| Minimum viable | 100 gal | Only for the recommended minimum groups, mature filtration, and close monitoring. |
| Recommended community | 150 gal | Adds territory, swimming room, and a safer buffer for behavior. |
| Low-conflict setup | 180 gal | Best when keeping larger schools or reducing territorial pressure. |