Hyaline cardinalfish

Where they come from

Hyaline cardinalfish (Foa hyalina) come from Indo-Pacific reefs. Think sheltered reef slopes and rubble zones where the light is a little broken up and there are lots of little hidey-holes. They are one of those small, see-through cardinals that look almost unreal under actinics.

Because they are naturally a bit cryptic and small-mouthed, most of the challenge with this species is getting them settled, eating reliably, and not outcompeted.

Setting up their tank

If you try to keep these like you would a bolder cardinal or a clownfish, you will probably just end up with a fish you never see that slowly loses weight. Give them a calm tank with structure and predictable routine.

• Tank size: I would treat 20-30 gallons as a practical minimum for a small group, bigger is easier for stable water and calmer social spacing.
• Aquascape: Lots of small caves, overhangs, and branching rock. Rubble piles and shaded pockets help them feel "invisible" (which is what they want).
• Flow: Moderate. They do not need a blender, but they also do poorly in stagnant corners where food just rots.
• Lighting: They handle normal reef lighting, but give them shaded areas so they can pick a comfort zone.
• Filtration: Strong enough to keep nutrients in check, since you will likely feed heavier and more often than average.

They do best in a mature system where pods and other micro-life are already present. That background "snack supply" buys you time while they figure out prepared foods.

What to feed them

This is the make-or-break section. Foa hyalina are micro-predators with small mouths, and they are not great at rushing a feeding frenzy. You want small foods, offered often, and you want to watch their bellies, not just assume they are eating.

• Best starters: Live baby brine, live copepods, or live enriched foods to kickstart feeding response.
• Frozen staples: Mysis (the smaller stuff), finely chopped krill, Cyclops, calanus, enriched brine, and good reef blends with tiny particles.
• Prepared foods: Tiny sinking pellets can work once they are confident, but do not expect pellets to be day-1 food.

Target feeding helps a lot. I like using a turkey baster or a long pipette to gently cloud food near their hiding area, not blasting them in the face. If you have faster fish, distract them on the other side of the tank first.

How they behave and who they get along with

They are shy, hover-y cardinals. Once settled, you will see them hanging midwater near rockwork, usually facing into the flow, picking at passing food. They are not "interactive" like a clownfish, but they are really cool in a subtle way.

• Best tankmates: Peaceful fish that do not dominate feeding time (small gobies, blennies, assessors, quieter wrasses, some dwarf angels if the tank is calm and feeding is controlled).
• Avoid: Aggressive dottybacks, pushy damsels, big wrasses that vacuum food, hawkfish that perch-and-pounce, and anything that turns feeding into a sport.
• Inverts: Generally fine with standard reef inverts, but tiny ornamental shrimp or very small new shrimp may be at risk if they fit in the mouth.

Grouping can work, but it depends on your space and the individuals. A small group in a bigger tank often makes them bolder. In tighter quarters, you can see bullying and one fish getting pushed off food.

Breeding tips

Like many cardinalfish, they are very likely mouthbrooders (male holds the eggs). In home tanks it is possible to see a male with a "puffy" mouth that stops eating for a bit. Actually raising the fry is the hard part because the babies are tiny and need very small live foods.

• If you spot a holding male: Keep the tank calm and do not chase him. Sudden stress can make him spit the clutch.
• Fry food: You will likely need rotifers first, then nauplii and copepods as they grow. Have cultures going before you try to raise any.
• Collection: A dimly lit fry trap near where the male hangs out can sometimes catch released fry without tearing the tank apart.

Common problems to watch for

Most losses with this species are slow-burn issues: they look fine for a couple weeks, then you realize they have been missing meals the whole time. The other big risk is rough shipping and parasite pressure on already-stressed fish.

• Starvation and pinched belly: The #1 problem. Watch body mass behind the head and along the back, not just whether they dart at food.
• Getting bullied off food: Even "peaceful" fish can outcompete them just by being faster.
• Crypt and other external parasites: Flashing, rapid breathing, hiding more than usual, and refusal to eat are red flags.
• Bacterial issues after shipping: Frayed fins, cloudy eyes, and lethargy can show up if they arrived banged up.
• Jumping: Cardinals can jump when spooked, especially in a new tank or during night scares.

A tight lid helps, and so does managing nighttime spooks (a dim room light for a bit after lights-out, or ramping down your lights if your setup allows it). Once they are settled and feeding well, they are a lot less nerve-y.