Philippine cardinalfish
Ostorhinchus mydrus
Also known as: Philippine mouthbrooding cardinalfish
Ostorhinchus mydrus is a little marine cardinalfish from the Philippines that hangs around coral reefs and comes alive at night. Like a lot of cardinals, its claim to fame is the male carrying the eggs in his mouth - super cool behavior if you ever get to see a pair settle in and spawn.
The Philippine cardinalfish exhibits striking red-orange coloration, with a distinctive black spot near the base of its dorsal fin.
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Quick Facts
Size
unknown
Temperament
Peaceful
Difficulty
Intermediate
Min Tank Size
20 gallons
Lifespan
unknown
Origin
Western Central Pacific (Philippines)
Diet
Carnivore/planktivore - small meaty foods like mysis, copepods, enriched brine, finely chopped seafood
Water Parameters
24-28°C
8.1-8.4
8-12 dGH
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Calculate heater sizeCare Notes
- Give them lots of cover - branching rock, caves, and a couple overhangs. They chill in the shadows and look stressed when the tank is too open and bright.
- Keep salinity steady around 1.025 (35 ppt) and temp about 76-79F; they do not like swings. Aim for nitrate under ~20 ppm and keep ammonia/nitrite at zero, since they are slow, steady breathers when something is off.
- Feed small meaty stuff: mysis, chopped shrimp, calanus, enriched brine, and good pellets once they recognize them. They are dusk feeders, so hit them right after lights dim (or in the evening) so faster fish do not steal everything.
- They do best in a calm community with other non-bullying fish - gobies, blennies, small wrasses, firefish, and peaceful tangs are fine. Skip aggressive dottybacks, big hawkfish, and any trigger that thinks 'snack size.'
- If you keep more than one, add them together and give multiple hiding spots so one does not claim the whole cave. In tight tanks, a pushy dominant fish can keep the others pinned in a corner.
- Quarantine if you can - they can show up with ich/velvet and they do not handle heavy parasite loads well. Watch for rapid breathing and hiding in the open, and treat early rather than waiting it out.
- Breeding is cool: the male mouthbroods the eggs, so you will see him stop eating and keep his mouth kind of 'full.' If you want fry, move him to a quiet tank near hatch time because the larvae are tiny and need rotifers/pods right away.
Compatibility
Good Tankmates
- Other small peaceful cardinals and similar gentle schoolers (like Pajama or Banggai cardinals). Philippine cardinals are pretty laid-back and usually do great in a calm group as long as nobody is bullying them.
- Small gobies (neon goby, clown goby, watchman goby). They mind their own business and do not outcompete the cardinals at feeding time if you spread the food around.
- Blennies with a chill attitude (tailspot blenny, bicolor blenny in a roomy tank). Good personality match and they use different parts of the tank, so they are not in each others faces.
- Peaceful wrasses like a flasher wrasse or fairy wrasse. Active but not usually mean, and they do not treat cardinals like a snack if the cardinal is decent sized.
- Small, non-aggressive clownfish (ocellaris or percula, especially a smaller pair). They can be a bit pushy near their spot, but most of the time they ignore cardinals and it works out fine.
- Reef-safe dartfish/firefish and similar shy swimmers. Same vibe: peaceful, a little timid, and they appreciate a tank without drama.
Avoid
- Dottybacks (especially orchid dottyback) and other territorial cave-claimers. They can harass cardinals nonstop, and the cardinal will just sulk in a corner and slowly waste away.
- Big or trigger-happy predators like lionfish, groupers, larger hawkfish. If it can fit the cardinal in its mouth, eventually it will. Cardinals are slow and easy targets at night.
- Mean damsels and other scrappy bullies (some Chrysiptera damsels, domino damsel). They love to pick on calm fish and will keep cardinals stressed and hiding.
- Nippy, aggressive wrasses (sixline in a smaller tank is the classic problem). They can turn into little torpedoes that chase anything calm, especially once they settle in.
Where they come from
Philippine cardinalfish (Ostorhinchus mydrus) are little reef hangers from the Philippines and nearby Indo-Pacific areas. Think sheltered lagoons, rubble zones, and reef edges where they can tuck into branching coral or rock crevices and hover in the shade.
They are a classic cardinalfish in attitude: calm, a bit shy at first, and way more active once the lights dim.
Setting up their tank
Give them structure and darker pockets. If the aquascape is just a bright open rock pile, they tend to stay jumpy and hide all day. If you build caves, overhangs, and a few tight crevices, you will see them settle in and hover out in the open more.
- Tank size: 20-30 gallons works for a pair or small group if you have lots of rockwork; bigger is easier if you want a group
- Aquascape: caves and overhangs, plus a couple of deep hiding spots they can claim
- Flow: moderate is fine, but make sure there are calm zones behind rock where they can hover
- Lighting: they do fine under reef lights, but they appreciate shaded areas
Cover the tank. Cardinalfish are notorious jumpers, especially the first week or two, and especially if you spook them during feeding or maintenance.
Water numbers do not need to be fancy, just stable. Aim for typical reef salinity (around 1.025-1.026), temps in the upper 70s F, and keep nitrate from creeping into the gross zone. What they hate is swings - quick salinity changes from top-off mistakes, or a tank that goes through mini-cycles.
What to feed them
These guys are planktivores. In the wild they pick tiny meaty stuff out of the water column, and in a tank they do best with small, frequent meaty feedings.
- Frozen: mysis (smaller pieces), brine shrimp (better if enriched), calanus, cyclops, finely chopped krill
- Prepared: small marine pellets and micro-pellets once they recognize them as food
- Live (great for new or shy fish): live brine, copepods, blackworms (if you use them, rinse well and do not overdo it)
If yours ignores pellets at first, do a little "follow the leader" trick: feed frozen first so they start snapping, then sneak in a few pellets in the same flow path. Most learn fast once they associate the spot with food.
Watch during feeding. They are not aggressive eaters, so if you keep them with pigs (big wrasses, dottybacks, pushy clowns), they can get outcompeted and slowly lose weight even though you swear you are feeding plenty.
How they behave and who they get along with
They are peaceful and kind of hover-y, usually hanging in the midwater near their chosen cave. They get bolder once they learn your routine, but they are still a "hang back and watch" fish.
You can keep them singly, as a pair, or in a small group. Groups look awesome, but in smaller tanks you can get squabbles as they sort out pecking order. More rock cover helps a lot.
- Good tankmates: gobies, blennies, smaller tangs in bigger tanks, fairy/flasher wrasses, peaceful reef fish
- Use caution with: dottybacks, big damsels, hawkfish, large wrasses, anything that bullies or feeds like a vacuum
- Avoid: predators with big mouths (groupers, lionfish, some big hawkfish) - cardinals are bite-sized
Cardinalfish are often more active at dusk and after lights out. If you only watch the tank in the brightest part of the day, you might think they are "boring" when they are really just waiting for low light.
Breeding tips
Like a lot of Ostorhinchus, they are mouthbrooders. The male typically holds the egg mass in his mouth until the fry are ready. If you see a fish with a "full jaw" that refuses food for days to weeks, that is usually what is going on.
- Keep a well-fed, calm pair and lots of cover. Stress and harassment can make the male spit the clutch.
- If you want fry, move the holding male (or the whole pair) to a quiet breeder tank with gentle filtration (sponge filter).
- Have tiny food ready: rotifers, baby brine, and/or copepod nauplii depending on fry size and what they will take.
Do not keep trying to "check" the male by netting him or shining lights in his face. Mouthbrooders can bail out and swallow or spit eggs if you hassle them.
Common problems to watch for
Most issues with Philippine cardinalfish come down to stress, shipping, and not getting enough food long-term. They can look fine for weeks and then slowly waste away if they are losing the feeding competition.
- Jumping: usually early on or after a scare - use a lid and keep lights changes gradual if you can
- Not eating at first: common with new arrivals - offer small meaty frozen or live foods and feed in the same spot daily
- Getting bullied: they will hide, stop feeding, and fade - rearrange rock, add cover, or move the bully
- Weight loss: look for a pinched belly - feed smaller foods more often and make sure they actually get bites
- Marine ich/velvet risk: cardinals are not "immune" - quarantine if you can, and do not add them to a tank mid-outbreak
Rapid breathing, staying out in the open looking panicked, or a sudden "dusty" look can mean velvet. Do not wait it out. Get the fish to a hospital setup and act fast.
If you nail three things - lots of cover, a calm community, and regular small meaty feedings - these fish are pretty forgiving and a really nice change of pace from the always-on zoomy reef fish.
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