Oblique-swimming triplefin

Where they come from

Oblique-swimming triplefins (Forsterygion maryannae) are little reef and rock-dwelling fish from New Zealand waters. Think surge-y rocky coastline, kelp edges, and lots of cracks and low turf algae to scoot around in. They live their whole lives close to structure, and they act like it in a tank.

If you're used to flashy open-water fish, triplefins feel more like a saltwater goby that never signed the goby rulebook. They perch, hop, and do these quick dart-and-freeze moves. Super cool, but they ask more of you than their size suggests.

Setting up their tank

Build the tank around rocks first, not around swimming space. They want a maze of small caves, narrow ledges, and shaded spots where they can sit with their belly touching something solid. If the rockwork is too open, they stay nervous and you'll barely see them.

• Tank size: bigger is easier, but footprint matters more than height. I'd treat 20-30 gallons as a practical starting point for a single fish, larger if you want multiples.
• Rockwork: lots of small crevices and tight overhangs. Think "stacked rubble" look rather than a couple big boulders.
• Flow: moderate with some messy, turbulent spots. They come from areas with surge, but they still need calm perches.
• Lighting: they do fine under reef lighting, but give shaded pockets so they can opt out.
• Lid: tight-fitting. They can launch when spooked, especially at night or during acclimation.

Filtration-wise, treat them like a small predator that eats messy foods. You want a strong skimmer, good mechanical filtration you actually change, and stable salinity. They're not forgiving if your tank swings around because you skipped top-off for a weekend.

What to feed them

Plan on feeding like you're keeping a picky micro-predator. In the wild they're picking off tiny crustaceans and other small prey. In captivity, the biggest hurdle is getting steady, confident feeding without competition.

• Best starters: live pods (Tigriopus, Tisbe), live enriched brine, small live mysis if you can get it.
• Frozen staples once they're going: mysis (smaller sizes), finely chopped krill, calanus, enriched brine, roe/eggs in tiny amounts.
• Dry food: some individuals learn small sinking pellets, but don't count on it. If they take it, great - still rotate frozen foods.

I like target-feeding with a small pipette or turkey baster. Put the food right in their "territory" so bolder fish don't vacuum it up first. If you just broadcast feed in a mixed reef, the triplefin often loses the race and slowly wastes away while looking "fine" day to day.

How they behave and who they get along with

They're perchers and lurkers, but they're not passive. They can be territorial with other small bottom-associated fish, especially anything that wants the same ledges and holes. The oblique-swimming thing is real too - they'll angle themselves and sort of hover against rock faces.

• Good tankmates: calm, non-competitive fish that won't bully the rockwork (some smaller peaceful wrasses can be okay in the right temp range, gentle planktivores, tiny cardinals).
• Tankmates to avoid: aggressive dottybacks, pushy damsels, hawkfish, big wrasses, and anything that constantly hunts the rock like a determined sixline-type fish.
• With other triplefins: depends on space and personalities. Multiple can work in a larger, very "busy" rockscape, but expect posturing and chasing.

They settle in faster if you give them a quiet tank and predictable routines. Sudden hands-in-tank sessions, loud bumps, or blasting the lights on from dark can send them flying. I ramp lights or use room light first.

Breeding tips

Triplefins are neat breeders, but raising the babies is where the "expert" label really shows up. Males typically guard eggs laid in a sheltered spot (underside of a rock or inside a tight crevice). If you keep a compatible pair and the tank is stable, you may see guarding behavior and a little nest site getting cleaned.

• Give them nest options: small flat stones leaned to create an underside, tight caves, and rubble zones.
• Keep feeds heavy and frequent leading up to spawning attempts.
• If you find eggs, leave the guarding male alone. Stress can make him abandon the clutch.

Common problems to watch for

Most losses with triplefins come from a few predictable issues: temperature mismatch, slow starvation, and stress from tankmates. They can look okay right up until they don't, so you have to be a little obsessive with observation.

• Refusing food after introduction: common. Offer live pods/brine, dim the lights, and make sure no one is harassing it.
• Slow weight loss: usually competition at feeding time. Target-feed and cut back on fast, greedy tankmates.
• Jumping: happens during the first week and during nighttime startle events. Use a lid and cover any cable gaps.
• Ich/velvet sensitivity: small fish with stress are magnets for parasites. Quarantine is your friend, and avoid sudden temp/salinity swings.
• Bacterial mouth/fin issues after scuffles: rock-dwellers get scrapes. Clean water and quick response matter.

If you want to succeed with this species, treat it like a specialized temperate micro-predator, not a generic "small reef fish." Nail the temperature range for where your specimen comes from, feed like you mean it, and give it a rockscape full of little hideouts. Do that and you'll get to see all the fun triplefin behavior that makes them worth the effort.