Oluolus hatchetfish

Where they come from

Polyipnus oluolus is one of those deepwater hatchetfish from the Indo-Pacific. In the wild they spend their lives in dim, blue light and come up and down in the water column with the daily light cycle. That deep-reef, midwater lifestyle explains basically every headache and every cool thing about keeping them.

Setting up their tank

These are not "pretty reef fish" that cruise around your rockwork. Think of them like pelagic, low-light planktivores that want open water, gentle flow, and a calm room. If the tank is bright, busy, or boisterous, they tend to pin themselves in a corner and slowly fade.

• Tank size: I would not bother under 40-55 gallons, and bigger is genuinely easier. They do better with room to hover midwater.
• Mature system: stable, established tank. New tanks with daily swings are a rough ride for them.
• Aquascape: keep rock to the sides/back and leave a long open "runway" of water.
• Lighting: dim to moderate. Use lots of shaded areas and avoid blasting them with reef-grade LEDs.
• Flow: smooth, laminar-ish flow they can sit in. Avoid chaotic high-turbulence gyres pointed at the midwater zone.
• Cover: tight lid or mesh. Hatchetfish can and will jump, especially on first introduction or during night frights.
• Night lighting: a faint room light or moonlight helps reduce spooking if you have nighttime activity in the room.

Water numbers are the usual reef range, but the vibe matters more than chasing decimals. Keep it stable: 1.025-1.026 salinity, 76-78F, low nitrate, high oxygen. I also run a slightly wetter skimmer and point a little surface agitation their way because they live in oxygen-rich water in nature.

What to feed them

Feeding is where most people lose them. They are tiny-mouthed midwater hunters. If food is too big, sinks too fast, or only hits the bottom, they just wont engage. You want a steady stream of small stuff drifting through the water column.

• Best starters: live baby brine (enriched), live copepods, live adult brine in a pinch (enrich it), and small live mysis if they will take it.
• Frozen that usually works: calanus, finely chopped mysis, cyclops, roe, and small plankton blends.
• Prepared foods: some will learn micro-pellets, but dont count on it early on. Use pellets as the long-term bonus, not the plan.

Small, frequent feedings beat one big dump. Think 2-4 light feedings a day, especially the first month. Watch their bellies from the side: you want a slight rounding after meals. If they stay razor-flat, theyre not getting enough even if food is going in.

How they behave and who they get along with

Oluolus hatchetfish are shy and a bit "nervy" at first. Once settled, they hover in the water column and do these quick, precise darts at food. They are not rock pickers. If they hide constantly, something about the tank is stressing them out (usually light, flow, or tankmates).

• Best kept in a small group if you can source them (3-7). Singles can do okay, but groups settle faster in my experience.
• Good tankmates: calm planktivores and mellow community fish that dont compete like crazy at feeding time (small cardinals, peaceful anthias species, timid wrasses that arent hyper).
• Avoid: aggressive feeders (chromis gangs, big wrasses, tangs that bulldoze the water column), nippy fish, and anything that sees a small midwater fish as a snack.
• Corals/inverts: generally reef-safe, but they dont care about your corals. They care about the water column.

Breeding tips

Captive breeding is rare. They are deepwater, likely spawn pelagic eggs, and the larvae would be tiny and demanding even by marine larval standards. If youre set on trying, the best realistic goal is getting them comfortable enough to show natural behavior, not counting on a batch of babies.

• Keep a group and get them fat on a varied plankton diet for months.
• Run a consistent day-night schedule with a long dusk period (ramp lights down slowly).
• If you ever see spawning behavior, expect eggs/larvae to end up in filtration unless you plan for a dedicated larval setup with gentle overflow protection.

Common problems to watch for

Most issues come down to three things: shipping/collection damage, stress from the environment, and slow starvation.

• Buoyancy problems (floating, sinking, rolling): can be barotrauma or internal injury from collection. Sometimes they improve with time and calm conditions, sometimes they dont.
• Not eating: usually food size/type or too much competition. Try live foods and feed in lower flow.
• Night frights and jumping: sudden lights, shadows, or loud room activity can trigger panicked launches.
• Wasting away with a "normal" appetite: could be internal parasites. Quarantine is tricky with deepwater fish, but if youre experienced, consider proactive observation and targeted treatment rather than shotgun meds.
• Rapid breathing: often low oxygen, high CO2, or being pinned in strong turbulent flow. Increase surface agitation and check if they have a calm zone to hover in.
• Skin damage: they can abrade themselves if spooked and crashing into rockwork. Keep open water and avoid sharp coral points in their main swimming lane.