Southern lightfish
Ichthyococcus australis
Southern lightfish (Ichthyococcus australis) exhibit a slender body with translucent skin, featuring bioluminescent organs along their ventral surface.
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About the Southern lightfish
This is a deep-ocean little lightfish that lives way down in the dark and uses photophores (tiny light organs) for camouflage and signaling. It is a pelagic marine species from the southern hemisphere, and its whole vibe is "midwater stealth" rather than anything you would ever keep like a normal aquarium fish.
Quick Facts
Size
12 cm
Temperament
Peaceful
Difficulty
Expert
Min Tank Size
0 gallons
Lifespan
unknown
Origin
South Atlantic (southern hemisphere)
Diet
Carnivore (likely zooplankton and small pelagic prey)
Care Notes
- This is a deepwater lanternfish, so set the tank up like a blackout room: dim blue lighting, lots of shaded overhangs, and zero sudden light flips unless you want a stress spiral.
- Keep it cold for a marine tank - think 8-12 C (46-54 F) with rock-solid salinity around 1.025; warm swings and sloppy top-off will knock them down fast.
- They do best in a tall, high-flow, high-oxygen system with a lid (they can spook-jump), and you will want oversized skimming because heavy feeding gets messy quick.
- Feeding is the whole game: small meaty stuff only (live enriched copepods, mysis, krill dust, chopped shrimp) and target-feed after lights-out when they actually want to hunt.
- Avoid aggressive or fast pigs at feeding time (wrasses, damsels, most dottybacks) because the lightfish will just starve; calm coldwater-compatible tankmates are the only safe bet.
- Watch for barotrauma and general 'deepwater shipping damage' - trouble swimming, floating, or red patches usually means it came up too fast and may never fully recover.
- Breeding in captivity is basically a lottery ticket: they are pelagic spawners, and even if you get eggs, raising the tiny larvae means constant live plankton and pristine filtration without sucking the babies into pumps.
Compatibility
Good Tankmates
- Small, mellow midwater fish that stay in their own lane - think small anthias or chromis types (not the pushy, hyper ones). Lightfish are calm and do best when nobody is trying to run the tank.
- Other gentle, small deepwater-style planktivores - flashlight fish and other non-aggressive, similar-size oddballs. If they are not bitey and not huge, they usually ignore each other.
- Tiny, peaceful gobies (clown gobies, neon gobies, small shrimp gobies). They hang near rocks/sand and do not compete much with a lightfish cruising midwater.
- Firefish and dartfish (Nemateleotris). Same vibe - shy, peaceful, and they will not hassle a lightfish. Give lots of cover so nobody is forced out into the open.
- Calm blennies that are not territorial bruisers (like tailspot-style personalities). They perch and pick at stuff and usually leave a lightfish alone.
- Small, non-nippy cardinals (Banggai-style peaceful tankmates). Similar pace and they are not typically fin chewers or bullies.
Avoid
- Dottybacks (most of them) - they are little psychos for their size and will absolutely stress a peaceful lightfish, especially in tighter rockwork.
- Aggressive damsels and big attitude chromis groups - constant chasing and pecking order stuff. A lightfish will just get pinned into a corner and stop feeding.
- Hawkfish - they look cute until they decide anything small is lunch or they start body-checking shy fish off perches.
- Anything predatory or mouthy enough to swallow them or harass them - lionfish, groupers, big wrasses, triggers. If it can fit a lightfish in its mouth, it will eventually try.
Where they come from
Southern lightfish (Ichthyococcus australis) are deepwater lanternfish relatives from the Southern Ocean region. They live way down in the dark, doing daily vertical migrations - hanging deep by day and moving up at night to hunt tiny drifting prey.
That deep-sea lifestyle is the whole story with this species. They are built for cold, dim, high-pressure water, and we can only fake a slice of that in an aquarium. That is why they land firmly in "expert only" territory.
Be picky about sourcing. Most deepwater lightfish do poorly from rough collection and shipping, and many arrive already compromised. If you cannot get a specimen that is feeding in a holding system, I would pass.
Setting up their tank
Think cold, dark, quiet, and boring (by reef standards). A dedicated chilled marine system is the starting point. If your setup is a typical 78F reef tank with bright lights and busy fish, this is not the fish to "try anyway".
- Tank size: bigger helps stability. I would not bother under 55-75 gallons, and 100+ is nicer for keeping the environment steady.
- Temperature: cold-water range. Aim roughly 45-55F (7-13C) depending on what your supplier kept them at, and keep it stable.
- Lighting: very low. Ambient room light or dim blue-only periods. They spook easily under bright LEDs.
- Flow: gentle to moderate, no blasting. You want food to stay suspended without pinning the fish to a corner.
- Filtration: oversized biofiltration and aggressive oxygenation. Cold water holds oxygen, but these fish hate dirty water and shipping stress already hits their gills hard.
- Aquascape: minimal rockwork, lots of open water, and a few dark overhangs/PVC sections to break line of sight. Fine sand is optional.
Give them a long, calm acclimation. Dim the room, dim the tank, and let them settle. I have had the best luck leaving the lights off for the first day and feeding only after you see normal hovering and tracking behavior.
Pressure is the elephant in the room. You cannot replicate deep-sea pressure at home, so success hinges on getting individuals that tolerate surface pressure and then keeping everything else (temp, light, water quality, handling) extremely gentle.
What to feed them
They are micro-predators. In the wild they pick off copepods, small krill-type critters, and other zooplankton. Getting them onto prepared foods is the hardest part, and honestly, some never switch.
- Best starter foods: live copepods, live enriched Artemia (small), live mysids if you can get tiny ones, and other live planktonic foods.
- Frozen options to try once feeding: cyclops, finely chopped mysis, calanus, krill dust/roe, and small marine plankton blends.
- Feeding schedule: small meals 2-4 times per day at first. Their whole vibe is "nibble constantly," not "one big meal."
- How to present food: target feed into a gentle flow so it drifts past them. Dumping food in one spot usually just feeds the filter.
If you can culture copepods or keep a refugium feeding the display, it helps a lot. A "background" population of pods can be the difference between a picky lightfish hanging on vs. slowly fading.
Watch the belly and the back line. These fish can look "fine" right up until they are thin. If you are not seeing consistent feeding responses within the first week or two, intervene fast with more live food and less competition.
How they behave and who they get along with
They are shy, midwater drifters. Most of the time they hover and make short darting moves to grab food. They do not like commotion, tapping on glass, sudden light changes, or boisterous tankmates.
- Good tankmates: basically none, unless you are building a dedicated coldwater pelagic setup with other very calm, similarly sized deepwater species.
- Bad tankmates: anything fast, nippy, or competitive at feeding time (which is most common marine fish).
- Best setup: species-only or near species-only, with you controlling feeding so they always get their share.
Avoid warm-water "reef" companions entirely. Even if you could keep them together temperature-wise (you cannot), the activity level and feeding aggression will stress the lightfish into hiding and starving.
Breeding tips
Realistically, breeding Southern lightfish in home aquaria is not something you should plan on. They are open-water spawners in the deep, and their early life stages are tiny plankton that would need specialized live feeds and probably very different conditions than the adults.
If you ever see courtship-like behavior (more activity in dim periods, following, tight circling), treat it as a sign your fish are comfortable, not as a cue that babies are on the way. Keep the tank stable and keep food coming.
Common problems to watch for
- Starvation (most common): they refuse prepared foods or get outcompeted. You notice weight loss, hollow belly, less interest in drifting prey.
- Shipping and decompression damage: poor equilibrium, rapid breathing, inability to stay upright, refusal to feed from day one.
- Light stress: frantic dashing, hiding nonstop, crashing into glass after sudden lights-on.
- Water quality swings: ammonia/nitrite issues hit fast in cold systems if the biofilter is not mature. Also watch CO2/pH in tightly sealed cool rooms.
- Mechanical injury: they are delicate. Nets and rough handling can damage slime coat and fins quickly.
Run the tank for a long time before the fish arrives. A mature biofilter plus a steady supply of live pods is the closest thing you get to "cheat codes" with deepwater plankton eaters.
Do not chase them around the tank. If you have to move one, use a container method (cup or specimen box) instead of a net, keep it submerged, and keep the room dim.
Similar Species
Other marine peaceful species you might be interested in.
Banggai Cardinalfish
Pterapogon kauderni
Banggai cardinals just sort of hover like little underwater satellites, and the bold black bars with those long, polka-dotted fins look unreal under reef lighting. They're super chill most of the time, but once a pair forms you'll see real "fish drama," and the male will even mouthbrood the babies like a champ.
Blackbreast cardinalfish
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This is a tiny deepwater cardinalfish that was only described in 2016, and it stays around 3 cm long max. The cool calling-card is the dark "blackbreast" patch on the chest area and the fact that the males mouthbrood eggs like other cardinalfish, even though it comes from way deeper water than the usual reef tank cardinals.
Blackfin slatey
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This is a big Indo-West Pacific sweetlips/grunt that cruises reefs and hangs in caves, and it gets that cool yellow-and-silver look sprinkled with dark spots plus the really obvious black on the lower tail and the pelvic/anal fins. Juveniles show up in murkier estuary and silty reef areas, then the adults shift deeper and often sit in small groups until they go hunting at night. In aquariums its size is the whole story - it is a public-aquarium kind of fish once grown.
Blackspot razorfish
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This is one of the coolest "knife-bodied" wrasses - it hangs over open sand and, when it gets spooked or wants to sleep, it literally torpedoes straight into the sand. Give it a deep, fine sand bed and it will act totally different (and way more natural) than a typical rock-hugging reef wrasse. Adults are usually shy and cruisy with tankmates, but they are not forgiving about rough handling or sketchy setups.
Blueband goby
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This is that classic gold/yellow-headed sand-sifting goby with the little blue cheek stripe-always busy, always rearranging your sandbed. In a reef tank it'll spend the day taking mouthfuls of sand, filtering out tiny critters/foods, then "snowing" clean sand back out, and it'll usually claim a burrow area (often as a pair in the wild). It's super cool behavior-wise, but you really do need a mature tank with a proper sandbed and a lid because they can jump.
Bristletail Filefish (Aiptasia-Eating Filefish)
Acreichthys tomentosus
This little weirdo is one of my favorites because it's got that goofy filefish "face," a knack for wedging itself into rockwork, and a ton of personality once it settles in. People love them for the chance they'll snack on nuisance Aiptasia, but even when they're not on pest patrol they're just fun to watch cruise around and pick at stuff all day.
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Blue Green Chromis (Green Chromis)
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Bluespotted dottyback
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