Yes: Compatible.
No clashes under our rules: they share a temperature range, neither is a fin nipper or predator of the other, and their temperaments fit.
Temperature. Cardinal tetra does best at 23–27 °C (73–81 °F) and Harlequin rasbora at 22–25 °C (72–77 °F). They share 23–25 °C.
Water. Their pH ranges overlap at 6–6.
Temperament and size. Cardinal tetra is peaceful and grows to 3.5 cm; Harlequin rasbora is peaceful and grows to 4.5 cm. Both use the mid of the tank, so give them room and cover.
Tank size and groups. Plan on at least 20 US gallons, the larger of the two cautious minimums. Keep Cardinal tetra in a group of 8+. Keep Harlequin rasbora in a group of 8+. A starter mix of 8 Cardinal tetra and 8 Harlequin rasbora in 20 gallons uses about 81% of the tank’s bioload capacity under our rules and grades A.
Keeping them together
- Feeding: both feed in open water. Feed small amounts two or three times a day so the slower eater gets its share.
- Aquascape: both are calmer with cover. Plants and floating cover make shy fish come out more, not less.
- Group size: a full group is not optional. A schooling fish kept in too small a group is stressed and more likely to nip or hide.
- Adding them: add the calmer species first and let it settle for a week or two before adding the other, and only once your tank has cycled.
Open this pair in the checker Add your other fish to grade the whole tank.
Quick facts
| Cardinal tetra | Harlequin rasbora | |
|---|---|---|
| Adult size | 3.5 cm | 4.5 cm |
| Min tank | 20 gal | 15 gal |
| Temperature | 23–27 °C | 22–25 °C |
| pH | 4.6–6 | 6–7 |
| Temperament | peaceful | peaceful |
| Group size | 8+ | 8+ |
Data from the Species Ledger, checked November 9, 2026, with at least two sources per species. Cautious community guidelines, not guarantees: individual fish vary. Not veterinary advice. Disagree? Tell us.