Shortspine cardinalfish

Where they come from

Shortspine cardinalfish (Ostorhinchus brevispinis) are Indo-Pacific reef fish that spend a lot of their time tucked into structure - think caves, ledges, and branching coral where the light is lower and the current breaks up. In the wild they hang in small groups and let the reef do the heavy lifting for shelter.

That background matters because in a bright, bare aquarium they usually look stressed and act like they are late to their own hiding spot all day.

Setting up their tank

I treat these like a shy, structure-dependent fish that also needs clean, stable saltwater. They are doable, but they punish sloppy setups, which is why I call them advanced.

• Tank size: 30+ gallons for a single or pair, bigger if you want a small group (and you will want the extra rockwork).
• Rockwork: lots of caves, overhangs, and tight crevices. Give them a few separate "parking spots" so one fish is not forced to share.
• Lighting: they do fine under reef lights, but add shaded areas. Floating macro or smart rock placement helps a ton.
• Flow: moderate, not blasting the spots they want to hover in. They like slack zones behind rock.
• Water stability: keep salinity steady (1.025-ish) and avoid swings. They do way better in mature tanks than brand new ones.

What to feed them

Most shortspine-type cardinals are crepuscular feeders - they get bolder around dusk and in lower light. In a bright tank, they may ignore food until they feel safe. Once they recognize you and the feeding routine, they usually turn into solid eaters.

• Great staples: enriched frozen mysis, finely chopped krill, calanus, brine shrimp (enriched), and quality small marine pellets once they accept them.
• If they are picky: live foods like live brine (enriched) or copepods can get them going, then transition to frozen.
• How often: small portions 1-2 times a day beats one big dump. They are designed for frequent little meals.
• Enrichment: soak frozen foods in a vitamin/HUFA supplement now and then. It helps with long-term conditioning.

How they behave and who they get along with

Shortspine cardinalfish are generally peaceful, but they are not "tough". They do best with calm tankmates that will not outcompete them at feeding time. Expect them to claim a favorite cave and hang there, especially during the day.

• Good tankmates: other peaceful reef fish (gobies, blennies, firefish with enough cover, fairy/flasher wrasses), cleaner shrimp, most reef-safe inverts.
• Use caution: dottybacks, larger hawkfish, aggressive damsels, and anything that rushes food like a maniac.
• Groups: they can be kept in small groups if the tank is big and the rockwork breaks line-of-sight. In tight quarters, you can see bullying and one fish getting pinned into a corner.

Breeding tips

Cardinalfish breeding is one of the cooler things you can witness in a marine tank. Like many cardinals, the male typically mouthbroods the eggs. You will notice a fish (usually the male) hanging back and not eating much, with a slightly "full" looking mouth.

• Conditioning: feed heavier with a variety of meaty foods and keep the tank calm. Spawning attempts often follow a stretch of stable conditions.
• Watch the mouthbrooder: once he is holding, avoid netting or chasing him. Stress can make him spit the clutch.
• Hatching: depending on species and temperature, you are often looking at roughly a couple weeks of holding. The fry are usually tiny and need very small live food (rotifers/copepod nauplii) right away.
• Best move: if you get a holding male and you are serious about raising fry, move him to a quiet, seasoned rearing setup before hatch - but only if you can do it without a rodeo.

Common problems to watch for

Most issues I see with these come down to stress: too much light with nowhere to hide, aggressive or hyper tankmates, or unstable water after a rushed setup. Stress turns into disease and feeding problems pretty quickly.

• Not eating: usually new-fish nerves or being outcompeted. Try feeding at dusk, use smaller foods, and make sure they have a secure hide.
• Jumping: almost always happens during acclimation week or after a scare. Cover the tank and keep sudden movements low.
• Ich/velvet: cardinals can be sensitive, and symptoms can show up fast under stress. Quarantine new fish if you can, and do not ignore early flashing or heavy breathing.
• Gill irritation from ammonia: they do not forgive "new tank" hiccups. Test, and do not add them to an immature system.
• Wasting away: can be internal parasites or chronic underfeeding in a busy community tank. Make sure they are actually getting food, not just watching it go by.