Compatibility Overview
Harlequin Rasbora 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 | Harlequin Rasbora | Koi | Verdict |
|---|---|---|---|
| Temperature | 72–81°F | 50–75°F | ⚠️ 72–75°F (narrow) |
| pH Range | 5.5–7.5 | 7.0–8.5 | ⚠️ 7.0–7.5 (narrow) |
| Temperament | peaceful | peaceful | ✅ Compatible |
| Adult Size | 1.75 in | 24 in | ❌ 14× difference |
| Min. Group | 8+ | 2+ | See below |
| Min. Tank Size | 100 gallons (for both species) | — | |
✅ Compatibility Positives
❌ Incompatibility Reasons
Species Profiles
Harlequin Rasbora — Trigonostigma heteromorpha
Active mid-level schooling fish with a distinctive black triangular patch. One of the best all-around community tank fish.
→ Full Harlequin Rasbora 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 Harlequin Rasbora live with Koi?
Generally not recommended. Harlequin Rasbora 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 Harlequin Rasbora and Koi together?
A minimum of 100 gallons is recommended for keeping Harlequin Rasbora and Koi together. A larger tank improves water stability and reduces territorial conflict.
What temperature suits both Harlequin Rasbora and Koi?
72–75°F (narrow)
Is a Harlequin Rasbora aggressive toward a Koi?
Harlequin Rasbora is peaceful and Koi is peaceful. Harlequin Rasbora requires tropical water; Koi requires cold water. These environments are incompatible.