Polynesian chromis

Where they come from

Polynesian chromis (Pycnochromis bami) are little damselfish from the tropical Pacific - think clear, surgey reef slopes and outer reef areas around Polynesia. In the wild they hang in the water column above the reef, picking plankton out of the current and ducking back to rock when something big cruises by.

That wild lifestyle explains most of the headaches people run into: they want oxygen, flow, open swimming room, and a safe bolt-hole. If you try to keep them like a lazy lagoon fish, they usually fade out on you.

Setting up their tank

I treat these like a small, fast reef schooling fish that also needs security. Give them open water up front, and a real rock structure with caves and crevices behind it. They spend a lot of time midwater, but they sleep and panic-dash into rock.

• Tank size: I would not do them in a nano. A single can work in a 30-40 gallon, but a group is happier in 55+ gallons with length to cruise.
• Aquascape: one main reefy rock pile with lots of holes, plus some broken sight lines. If you build one flat wall, the boss fish can police the whole tank.
• Flow: moderate to strong, with some chaotic movement. They love to sit in the current and snap at food.
• Filtration: oversized skimmer and good mechanical filtration help a lot because you will feed heavy.
• Lighting: whatever your reef runs. They do not care, but they appreciate shaded rockwork to retreat to.

If you are adding them to an established reef, I like to add them after the tank is mature and pod/plankton life is going, but before you have a tank full of pushy fish. They settle faster when they are not immediately getting bullied off food.

What to feed them

They are planktivores. In practice that means lots of small foods, offered more than once a day. The biggest mistake is feeding them like a clownfish - one big meal and done. They will eat it, then slowly lose weight anyway.

• Frozen: mysis (smaller pieces), finely chopped krill, calanus, brine plus, roe/eggs, and good mixed "reef plankton" blends.
• Dry: small pellets (0.5-1 mm), tiny granules, and flakes that stay suspended a bit.
• Live (great for new/shy fish): enriched live brine, copepods, or live baby brine to get them eating aggressively.

Watch their bellies. A healthy chromis has a gently rounded belly after a meal and does not look pinched behind the head. If they look "knife thin" even though they are eating, step up feeding frequency and consider parasites.

How they behave and who they get along with

They are one of those fish that looks like it should be peaceful, but the social side can get messy. In a group, they often form a pecking order. In a small tank or a group that is too small, that can turn into one fish getting harassed until it hides and wastes away.

• Best kept: singly, or in a larger group in a bigger tank. Odd numbers can help, but space and rockwork matter more than the number.
• Good tankmates: other reef-safe fish that are not hyper-territorial in the midwater (fairy wrasses, some anthias, many gobies, tangs in larger tanks).
• Use caution: dottybacks, hawkfish, established clown pairs, and most aggressive damsels - they can keep chromis stressed and underfed.
• Avoid: anything that will repeatedly rush the water column (big aggressive wrasses, triggers) unless the tank is very large and you know the personalities.

They are reef-safe with corals and inverts. The only time I have seen them bother anything is incidental - snapping at food near a coral or picking at the water column above a rock.

Breeding tips

They can spawn in captivity, though raising the babies is a whole other hobby. In a stable reef with a bonded pair, the male will usually pick a rock surface or a little cave ceiling and clean it. Spawning tends to happen when they are well-fed and the tank is calm.

• You may notice: cleaning a spot, chasing other fish away from that area, and the pair staying tight together.
• Eggs: typically laid on a hard surface and guarded. The male fans them.
• If you want to try raising larvae: you will need a separate larval setup and tiny live foods (rotifers, then nauplii). Most people let the display fish handle the eggs and just enjoy the behavior.

Common problems to watch for

Most failures with Polynesian chromis look like "mysterious" slow decline. Usually it is one of three things: not enough food per day, social stress, or disease introduced during a rough acclimation.

• Wasting away despite eating: often internal parasites. Quarantine and treat if you can, and do not wait until they are paper-thin.
• Uronema-like issues (common in chromis in general): rapid decline, sores, redness, or sudden deaths after stress. A good QT protocol and buying robust fish helps a lot.
• Bullying in groups: torn fins, hiding, missing meals. Fix with more space, more rock breaks, heavier broadcast feeding, or rehoming the aggressor.
• Jumping: they are quick and spook easily. A lid saves lives.
• Oxygen stress: heavy breathing at the surface, especially at night. Add surface agitation and keep the skimmer running.

If you keep water stable, feed small foods often, and give them room to be midwater fish, they are awesome. They bring that constant, busy reef motion without being a coral pest - you just have to meet them halfway.