Highfin threadsail
Hime diactithrix
The Highfin threadsail features a distinctive elongated dorsal fin and a body marked by iridescent blue and green hues.
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About the Highfin threadsail
This is a deepwater little threadsail/flagfin from the Western Pacific that lives way down on the continental shelf. Its whole vibe is that tall, sail-like dorsal fin with warm orange spotting and bands, but because it comes from around 200-300 m it is basically never an aquarium fish in any normal sense.
Also known as
Quick Facts
Size
17 cm (standard length)
Temperament
Peaceful
Difficulty
Expert
Min Tank Size
180 gallons
Lifespan
unknown
Origin
Western Pacific
Diet
Carnivore - likely small fishes/crustaceans and other benthic inverts
Water Parameters
6-12°C
8-8.4
8-12 dGH
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This species needs 6-12°C in a 180 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Give it a big, calm tank with lots of open water and a few caves - these guys spook easy and will pace if they feel boxed in. Tight lids are non-negotiable because they can launch when startled.
- Keep water rock-solid: 1.025-1.026 salinity, 24-26 C (75-79 F), pH 8.1-8.4, and nitrates as close to zero as you can realistically keep them. They react fast to swings, so an ATO and steady alkalinity matter more than chasing fancy numbers.
- Feed small meaty stuff often - think enriched mysis, finely chopped shrimp, and quality marine pellets once its taking prepared foods. New ones can be picky, so start with live or frozen (and soak in vitamins) until it locks onto your routine.
- Avoid pushy tankmates: no aggressive tangs, big wrasses, triggers, or anything that will outcompete it at feeding time. Peaceful midwater fish and non-nippy species are the move because those fins get shredded by fin-nippers.
- Go heavy on oxygen and flow without blasting it directly - strong surface agitation and a skimmer help a lot. They do worse in 'pretty but still' tanks and can get lethargic in low O2.
- Quarantine if you can because they are magnets for marine ich and flukes, and stress makes it blow up fast. Watch for rapid breathing, flashing, and frayed thread fins - those are usually your early warnings.
- Breeding in home tanks is basically a lottery; if you ever see a pair doing dusk spawning rises, keep the lights ramp gentle and cover intakes. Eggs and larvae are tiny and get wiped out by filtration and hungry tankmates almost immediately.
Compatibility
Good Tankmates
- Small, calm gobies (neon gobies, clown gobies, watchman gobies). Threadsails are super mellow and these guys mind their own business, so its usually zero drama.
- Peaceful blennies like tailspot or bicolor blennies. They hang on the rocks, do their blenny thing, and dont hassle the threadsail.
- Small, well-behaved clownfish pairs (ocellaris/percula). They can be a little pushy right around their anemone or corner, but in a normal-sized tank its generally fine.
- Firefish (dartfish) and other shy, gentle swimmers. Similar vibe - just make sure everybody has bolt-holes so the threadsail doesnt get outcompeted at feeding time.
- Banggai or pajama cardinals. Great peaceful midwater fish that dont nip fins and dont freak out the threadsail.
- Reef-safe wrasses that are on the calmer side (like a possum wrasse). Active but not typically fin-nippy, and they wont bulldoze a timid threadsail.
Avoid
- Fin-nippers and pushy semi-aggressive fish (many damsels, esp. domino/three-stripe types). They will harass a threadsail and shred those long fins for fun.
- Dottybacks (royal, orchid, etc.) in smaller tanks. They look cute but can be little terrors, especially with timid fish and new additions.
- Big, assertive wrasses and hogfish (six-line can be a coin flip, and the bigger wrasses are worse). If its known for bullying or constant chasing, it usually ends badly.
- Hawkfish (like flame hawkfish) and other perch-and-pounce bullies. They can pick on slow, fancy-finned fish and stress them out nonstop.
Where they come from
Highfin threadsails (Hime diactithrix) are one of those oddball, deep-reef-ish marine fish that show up rarely and make you stop scrolling. The long fin streamers are the giveaway. In the wild they are not hanging out in bright, shallow reef flats - think lower light, calmer pockets, and lots of vertical structure to tuck into.
If your fish was collected deep or handled rough in shipping, it may arrive looking fine but act "off" for days. Plan for a slow, gentle start instead of tossing it into a busy reef on day one.
Setting up their tank
This is an expert fish mostly because of acclimation and feeding, not because it needs some magical water number. Stable, mature, boring saltwater is what you want. If your tank swings 1-2 degrees daily, or salinity drifts because of lazy top-off, you are going to feel it with this species.
Give them a tank with plenty of rockwork that creates caves and overhangs, plus a couple of clear lanes to glide through. They like to hover and "hang" in a spot once they feel safe. A bare, open aquascape makes them nervous and they will burn energy pacing.
- Tank maturity: I would not try one in a brand-new system. You want a tank that has been steady for months, not weeks.
- Flow: moderate, not blasting. Aim for areas of gentler flow where it can rest.
- Lighting: they do better with shaded zones. If you run high-intensity reef lights, build ledges and caves so it can choose shade.
- Cover: a tight lid. Fish with streamers can still jump, especially during the first week.
Skip the "float-and-dump" style acclimation. Do a slow drip and match salinity closely. Shipping stress plus a quick salinity swing is a common one-two punch.
What to feed them
Feeding is where most people win or lose with this fish. Mine took food, but only once it stopped feeling hunted. The first few days I focused more on peace and routine than stuffing it full.
Start with small, meaty items that move in the water column. Think of what a shy, hovering fish can snick out of the flow without charging across the tank. After it is taking those confidently, you can widen the menu.
- Best starters: live or enriched frozen copepods, mysis (smaller pieces), finely chopped shrimp, calanus
- Also works: enriched brine (as a bridge food), roe/masago-style fish eggs, finely chopped clam
- Prepared: some will take high-quality micro pellets, but I treat pellets as "later" food, not day-one food
If it is shy, feed with pumps briefly lowered and use a turkey baster to place food up-current of its favorite hover spot. Once it learns that spot equals dinner, it gets bold fast.
Watch the streamers during feeding. Aggressive tankmates will nip them, and even a "friendly" clownfish can turn into a fin-tasting missile at mealtime.
How they behave and who they get along with
They are not a brawler. Mine was calm, a little spooky, and liked to claim a specific ledge as home base. Once settled it spent more time in the open, but it still hated sudden chaos.
Pick tankmates that are polite at feeding time and not obsessed with long fins. You can keep it with other peaceful fish, but the vibe matters more than the species list.
- Good fits: smaller, calm wrasses (not bullies), gobies, blennies, cardinals, peaceful anthias if you can keep everyone fed
- Risky: dottybacks, larger hawkfish, aggressive damsels, big established clowns, most triggers, fin-nippy angels
- Inverts/coral: generally reef-safe in the "not a coral eater" sense, but stress in a packed SPS rave-tank is the real issue
Do not pair it with known fin nippers. Those streamers are basically an invitation, and once they get shredded the fish often stays stressed and stops eating.
Breeding tips
Breeding in home aquariums is not really a thing with this species. Even if you get a male/female pair (good luck) you are dealing with a fish that is touchy about stress and probably has a pelagic larval stage that is hard to raise without dedicated larval setups.
If you ever see courtship behavior (extended fin displays, circling, dusk activity changes), treat it as a sign your tank is stable and the fish feels secure, not as a project you are going to casually pull off.
Common problems to watch for
Most problems trace back to shipping stress, not eating early enough, and being housed with pushy fish. If you plan for a slow acclimation and a quiet first week, you dodge a lot of the heartbreak.
- Refusing food: usually stress or competition. Dim the lights, reduce flow, target feed, and remove aggressive feeders if needed.
- Fin damage: nipping or getting pinned to a strong pump intake. Use guards and keep an eye on "friendly" bullies.
- Crypt/velvet: this is a sensitive fish - quarantine is your friend, but keep it low-stress. Observe breathing rate and flashing.
- Bacterial fin rot after damage: torn streamers can get infected. Clean water, reduced stress, and early intervention helps a lot.
- Sudden hiding and heavy breathing: often ammonia/pH swing, temp swing, or a parasite bloom. Test first, then treat.
My personal rule with rare, touchy fish: have a calm, cycled QT ready before you buy it, even if you end up not using it. Scrambling after the fish arrives is how mistakes happen.
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