Flagfin cardinalfish

Where they come from

Flagfin cardinalfish (Jaydia truncata) are Indo-Pacific fish you will see around reefs and rubble zones, usually hanging in the shadows and hovering close to cover. They are one of those cardinals that look calm and tidy in the shop, then turn into little pigs at feeding time once they settle in.

Most of the ones in the hobby are wild-caught, so think in terms of a fish that appreciates stable water, a quiet first week, and plenty of hiding spots while it learns your schedule.

Setting up their tank

Give them a reef-style setup with lots of nooks and shaded areas. They are not a "needs open water" swimmer. Mine spent the first month posted up under an overhang and only ventured out when the lights dimmed.

• Tank size: 20-30 gallons for a single fish, 40+ if you want a small group
• Aquascape: rockwork with caves, overhangs, and a few tight crevices they can claim
• Flow: moderate is fine, but leave some calmer pockets behind the rocks
• Lighting: they do fine under bright reef lights as long as there are shadows to retreat into
• Filtration: typical reef filtration works well; they are messy eaters so plan for nutrients

Acclimation matters with this species. I have had the best luck with a slower drip acclimation and lights off the rest of the day. They are hardy once established, but they do not love big swings on day one.

What to feed them

They are carnivores and usually easy to convert to frozen, but the first few meals can be weird. In my tanks, they take small meaty foods best and they do better with two smaller feedings than one big dump.

• Frozen mysis (smaller pieces are easier for them to handle)
• Enriched brine shrimp (good for getting a new fish eating, not a staple by itself)
• Chopped krill or clam (sparingly, can be messy)
• Calanus or other small plankton foods
• Quality marine pellets once they recognize them as food

Watch them eat, not just "feed the tank." A confident tankmate can outcompete them, and then you are left wondering why the cardinal looks thin even though you feed daily.

How they behave and who they get along with

They are generally peaceful, hover-y fish. Think calm reef community vibes, not action fish. They do not bother corals and they do not pick fights, but they can be surprisingly territorial with other cardinals once they pick a favorite cave.

• Good tankmates: clownfish, gobies, blennies, fairy wrasses, peaceful tangs (in bigger tanks), reef-safe inverts
• Use caution: other cardinalfish, dottybacks, hawkfish, aggressive damsels, and pushy wrasses that steal food
• Avoid: predators that can swallow them (bigger groupers, lionfish, large hawkfish depending on size)

They are also more active at dusk. If your tank goes dead quiet at night, you will still catch them cruising once the lights ramp down.

Breeding tips

Like many cardinalfish, they are mouthbrooders. The male typically holds the eggs in his mouth until they hatch. You will notice he stops eating and his jaw looks a little "full" and he hangs in a safer corner.

• Condition the pair with frequent small meaty meals (mysis, calanus, enriched foods)
• Keep the tank calm during holding - chasing the male around can make him spit the clutch
• If you want to raise fry, you will need a separate setup and tiny live foods ready (rotifers first, then baby brine)
• Cover overflows and pumps - free-swimming babies get sucked up fast

Common problems to watch for

The biggest issues I see with flagfins are not exotic. It is the everyday stuff: stress from rough shipping, not getting enough food in a busy tank, and the usual marine parasite gauntlet.

• Not eating after purchase: often stress or being intimidated by tankmates
• Getting outcompeted at feeding time: they hover and pick, so faster fish can leave them skinny
• Marine ich or velvet: especially if added without quarantine
• Bacterial infections after shipping (frayed fins, cloudy eyes) if they got beat up in transit
• Jumping: less notorious than some fish, but still possible if spooked