Compatibility Overview
Koi and Red Eye Tetra 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 | Red Eye Tetra | Verdict |
|---|---|---|---|
| Temperature | 50–75°F | 72–79°F | ⚠️ 72–75°F (narrow) |
| pH Range | 7.0–8.5 | 6.0–7.0 | ⚠️ 7.0–7.0 (narrow) |
| Temperament | peaceful | aggressive | ❌ Risk |
| Adult Size | 24 in | 1.6 in | ❌ 15× difference |
| Min. Group | 2+ | 1+ | 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.
Red Eye Tetra — Aquarium species
The Red Eye Tetra is a popular freshwater fish known for its vibrant red eyes and sleek, silver body. They are generally peaceful and can be kept in a community tank with other non-aggressive species. Their moderate activity level makes them suitable for a variety of aquarium setups. With proper care, they can live up to 5 years.
Frequently Asked Questions
Can Koi live with Red Eye Tetra?
Generally not recommended. Koi and Red Eye Tetra 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 Red Eye Tetra together?
A minimum of 100 gallons is recommended for keeping Koi and Red Eye Tetra together. A larger tank improves water stability and reduces territorial conflict.
What temperature suits both Koi and Red Eye Tetra?
72–75°F (narrow)
Is a Koi aggressive toward a Red Eye Tetra?
Koi is peaceful and Red Eye Tetra is aggressive. These animals require different water types and cannot share one aquarium.
Related Compatibility Guides
Compatibility score breakdown
The overall score is weighted across water chemistry, behavior, adult size, swimming zone, and social needs. A score is a planning aid—not a guarantee for individual fish.
| Factor | Score |
|---|---|
| Temperature | 43/100 |
| Ph | 0/100 |
| Temperament | 30/100 |
| Adult Size | 8/100 |
| Swim Zone | 68/100 |
| Social Needs | 92/100 |
Tank size scenarios
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. |