Cardinal Tetra and Discus: Can They Live Together?
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Side-by-side comparison
| Factor | Cardinal Tetra | Discus | Assessment |
|---|---|---|---|
| Water temperature | 73–81°F | 82–88°F | No overlap |
| pH | 5.5–7 | 5.5–7 | Overlap 5.5–7 |
| Hardness | 1–8 dGH | 1–8 dGH | Overlap 1–8 dGH |
| Adult size | 2.0″ | 8.0″ | 4.0× difference |
| Temperament | Peaceful | Peaceful | Both peaceful |
| Swim level | Mid | Mid | Same zone |
| Minimum tank | 20 gal | 55 gal | Together: 55 gal+ |
Shared water parameters
- No shared temperature window.
- pH both tolerate: 5.5–7
- Hardness both tolerate: 1–8 dGH
- Recommended combined tank: 55 gallons or larger
Risks and things to watch
Size mismatch — predation risk
Discus reaches 8.0″ while Cardinal Tetra tops out at 2.0″. As a rough rule, anything that fits in another fish's mouth eventually will. This is the main reason this pairing needs care.
Temperature conflict
The comfortable ranges barely meet. Running one species at the edge of its range for months is a slow-burn health problem, not an instant one — which is why it often goes unnoticed.
Both occupy the mid zone
Competition for space and food is higher when two species share the same layer. Increase footprint rather than height.
Group requirements
Compatibility is not only about the two species getting along — each still needs its own social setup met inside the same tank.
- Cardinal Tetra — minimum group of 6. Budget tank space for the whole shoal, not one fish.
- Discus — minimum group of 5. Budget tank space for the whole shoal, not one fish.
Frequently asked questions
Can cardinal tetra and discus live together?
Not recommended. We do not recommend housing Cardinal Tetra with Discus. The conflicts below are structural — they are not solved by a bigger tank or more plants. Choose a different tank mate instead.
What size tank do cardinal tetra and discus need together?
Plan for at least 55 gallons. Cardinal Tetra alone needs 20 gallons and Discus needs 55, but combined stocking needs headroom for filtration and swimming space, not just volume.
What temperature suits both species?
There is no shared comfortable range: Cardinal Tetra needs 73–81°F and Discus needs 82–88°F. That alone rules the pairing out.
What pH works for both?
Aim for pH 5.5–7, which sits inside both species' comfortable range.
Will discus eat cardinal tetra?
It is a real risk. The size difference is 4.0×, and anything that fits in a fish's mouth is eventually treated as food.
How should the tank be arranged?
Use plants, wood and rock to break sight lines, keep open swimming space along the front, and add the more territorial species last so it cannot claim the whole tank before the other settles in.
Explore further
Assessment generated from our compatibility methodology: parameter overlap, adult size ratio, temperament and swimming zone. Reviewed by Marcus Rivera.