Threadfin Cardinalfish

The threadfin cardinalfish is recognized by its semi-transparent whitish body, the tiny blue spots scattered over it and the narrow, broken orange bars on the head and front of the body. Its bright blue eyes and tall first dorsal fin are its most striking features. It reaches about 6 cm.

It is smaller and more delicate than the Banggai and pajama cardinalfish, which have their own pages on this site, but it is easy to keep. Its peaceful nature and schooling behavior bring life to the middle layer of a calm reef aquarium. The fish sold are usually wild-caught.

It lives in the Indo-Pacific from the Red Sea to Samoa and Tonga, in sheltered and often turbid bays and lagoons at depths of 1–12 meters. It forms dense schools above branching Porites cylindrica corals, often mixed with other cardinalfish. It is a nocturnal plankton feeder and spends the day hovering around coral branches.

• Volume: at least 150 liters for a school of 5–7.
• Water: salinity 1.023–1.026 specific gravity, pH 8.1–8.4, temperature 24–27°C.
• Layout: branching corals or branching rock structures; shaded areas where it can shelter during the day.
• Flow: gentle to moderate flow; strong flow tires this small fish.
• Reef compatibility: reef safe; it leaves corals alone. It may eat very small shrimp larvae but leaves adult ornamental shrimp alone.

Carnivorous. Offer frozen mysis, brine shrimp, copepods and fine granules. Because it is a nocturnal species, an evening feeding before the lights dim is very helpful. Feed small amounts 2–3 times a day.

Peaceful and somewhat shy. Pairs form within the school, and during breeding a pair defends a small area from the others; these squabbles are rarely serious. Among fast, aggressive fish it cannot get to the food.

Good companions: clownfish, gobies, blennies, royal gramma, fairy wrasses, other peaceful cardinalfish, ornamental shrimp.

Avoid: predators such as lionfish and hawkfish, aggressive damsels, large dottybacks, large wrasses.

Kept as a school: at least 5, preferably 7 or more individuals should be added at the same time. One or two fish stay timid and hide. Over time, pairs become distinct within the school.

Cardinalfish are mouthbrooders: after the female releases her eggs, the male takes the egg mass into his mouth and carries it until it hatches. The male does not eat during this period. Spawning can be observed in the aquarium; raising the planktonic larvae requires live food cultures.

• New fish are sensitive to shipping stress; quarantine them in a calm tank.
• Marine white spot and velvet: 3–4 weeks of quarantine.
• Watch whether weaker individuals in the school are getting food.

There is no reliable external difference. A female carrying eggs has a fuller belly; a male carrying eggs in his mouth has a clearly swollen throat.