Johnston Island damsel
Plectroglyphidodon johnstonianus
The Johnston Island damsel exhibits a vibrant blue body with yellowish accents along the fins and a distinctive black spot at the base of the dorsal fin.
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About the Johnston Island damsel
This is one of those tough little reef damsels that acts like it owns the whole rock pile, especially once it settles in. Maxes out around 14 cm and will absolutely defend a favorite cave or coral head, but the blue eye and chunky "wide bar" look make it a really cool fish if you plan the tank around its attitude.
Also known as
Quick Facts
Size
14 cm
Temperament
Aggressive
Difficulty
Beginner
Min Tank Size
55 gallons
Lifespan
5-10 years
Origin
Indo-Pacific
Diet
Omnivore - pellets/flakes plus frozen meaty foods; will also graze algae
Water Parameters
22.2-25.6°C
8.1-8.4
8-12 dGH
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This species needs 22.2-25.6°C in a 55 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Give it real rockwork with lots of nooks and a couple of tight caves - once it picks a spot, it will defend that whole chunk of the tank like its paying rent.
- Reef salinity and stability matter more than chasing fancy numbers: keep it around 1.020-1.025, 72-78F, pH about 8.1-8.4, and keep ammonia/nitrite at zero with nitrates kept low.
- Feed small amounts 1-2 times a day and mix it up: pellets/flakes plus frozen mysis, brine, and chopped seafood, and toss in some spirulina or algae-based foods so it does not go full gremlin.
- Tankmates need to be confident and not easily bullied - think larger wrasses, tangs, rabbitfish, and sturdy clowns; avoid tiny gobies, firefish, and shy blennies unless the tank is big with lots of escape routes.
- Skip keeping it with other damsels unless the tank is large, because it tends to turn into nonstop territory drama, especially once it settles in.
- If you want it in a reef, watch your corals - this fish can decide a coral head is its home turf and may nip or annoy polyps just from constant hovering and defending.
- Breeding is classic damsel: they lay eggs on a cleaned rock near their cave and the male guards and fans them, which also means they get extra spicy during spawning and may start attacking your hands during maintenance.
Compatibility
Good Tankmates
- Tough semi-aggressive fish that can hold their ground - bigger wrasses (like a Melanurus or a sturdy Halichoeres), since they are fast, confident, and not easily bullied
- Hawkfish (flame hawk, longnose hawk) - they perch, they are bold, and they do not get pushed around much by a cranky damsel
- Rabbitfish (Foxface, Siganus spp.) - usually too big and chill for the damsel to seriously hassle, and they have that 'do not mess with me' vibe
- Bigger, attitude-ready angelfish (dwarf angels like coral beauty, flame angel) - not a guaranteed peace treaty, but they are generally scrappy enough to coexist in a roomy rockwork-heavy tank
- Dottybacks (orchid, springeri) - they are not pushovers and tend to stick to their own caves, so it is more of a standoff than a beatdown
- Other robust 'reef brawler' types like a one-spot foxface or a sturdy tang in a big tank - the damsel will still posture, but it usually cannot dominate the whole tank
Avoid
- Small peaceful fish like gobies and firefish - Johnston Island damsels love to claim rock and chase timid fish into hiding until they stop eating
- Clownfish (especially if they are smaller or newly added) - the damsel often treats them like intruders and will harass them nonstop around the rockwork
- Slow, gentle fish like cardinals and jawfish - they just do not have the speed or attitude to deal with repeated charging and nipping
- Other damsels and similar-shaped territorial fish in smaller tanks - usually turns into a turf war unless the tank is big and the rockwork breaks up sight lines
Where they come from
Johnston Island damsels (Plectroglyphidodon johnstonianus) come from the central Pacific around Johnston Atoll. In the wild they hang around shallow reef zones with lots of rock and rubble, picking at algae and staking out little territories like they own the place.
They are one of those fish that looks small and harmless in the store, but once they settle in they act like a true reef tough guy.
Setting up their tank
Think rockwork first, open water second. This species wants a home base and a few escape routes. If you give them one obvious "best cave" they will claim it and patrol a chunk of the tank around it.
- Tank size: I would not do one in less than 30 gallons, and 55+ is a lot easier if you want other fish too
- Rockwork: build a couple of separated rock piles or islands so other fish can keep their distance
- Flow: moderate to strong, like a typical reef tank (they handle it fine and it keeps algae and detritus from settling)
- Lighting: whatever suits your reef; they do not need special lighting but they will graze more with decent light on the rocks
- Water: stable salinity around 1.025 and reef-range temp (roughly mid-70s F)
If you are adding one to an established tank, rearranging a bit of rock right before introduction can cut down the "this is my turf" attitude. Not magic, but it helps.
What to feed them
These guys are not picky once they are settled. Mine always acted like a grazer that also happily takes meaty stuff. The biggest mistake I see is feeding only brine or only pellets and calling it done. Mix it up and they keep better color and attitude.
- Daily basics: a quality pellet or flake as a staple (small size so they can grab it easily)
- Frozen: mysis, brine (as a treat), chopped krill or clam, and a good reef blend
- Greens: spirulina flakes, algae-based pellets, and nori on a clip (they will pick at it)
- Tank food: they will also graze film algae and microgrowth on the rocks
Feed smaller amounts more than once if you can. A well-fed damsel is still territorial, but it is usually less likely to go looking for trouble all day.
How they behave and who they get along with
This is the part people either love or regret. Johnston Island damsels are bold, fast, and very sure of themselves. Once they pick a zone, they will chase anything that wanders too close. In a bigger tank it is manageable. In a smaller tank they can turn into the tank boss.
They usually do better with fish that do not look or act like other damsels and that can take a little posturing. Slow, gentle fish tend to get stressed out by constant drive-bys.
- Good matches: tangs (in appropriate tank sizes), wrasses that are quick and confident, larger clownfish, most dottybacks that hold their own
- Risky matches: other damsels (especially similar shape/color), small gobies that perch near the claimed rock, firefish, tiny blennies that want the same holes
- Usually fine with corals: yes, but they may rearrange or nip at algae around a frag if it is near their nest/territory
Do not count on "adding it last" to fix everything. It can still decide it owns the tank. Adding it last helps, but tank size and rock layout matter more.
Breeding tips
They will spawn in home aquariums, especially if you keep a bonded pair and the tank feels stable. Like a lot of damsels, they are substrate spawners. The male usually picks a spot on rock and keeps it clean, then guards the eggs hard.
- Pairing: buying two and letting them sort it out can get ugly in smaller tanks; a larger tank with lots of rock gives them a better shot
- Spawning site: a flat rock face or a cleaned patch near their home base
- Egg care: expect the guarding fish to be extra spicy for a week or so
- Raising fry: the hard part is food size - you will need rotifers and then baby brine, plus a separate rearing setup
Even if you do not plan to raise fry, spawning behavior is worth knowing because it often explains a sudden jump in aggression.
Common problems to watch for
Most issues I have seen with this species are not mysterious diseases, they are husbandry and social problems: stress from bullying (either them bullying others or getting harassed in a cramped tank), and the usual saltwater stuff that shows up when a fish is newly shipped.
- Aggression injuries: torn fins and missing scales on tankmates that wander into the territory
- Stress hiding: a new fish that never comes out usually needs more cover, less competition, or a calmer neighborhood
- Marine ich/velvet risk: especially on new arrivals; quarantine is your friend
- Starvation in new imports: a shy or beat-up fish can get outcompeted, so target feeding can help early on
If one fish is getting pinned in a corner or not allowed to eat, do not wait it out. Move rock to break sight lines, use an acclimation box, or pull one of the fish. Damsels can harass a tankmate to death faster than people expect.
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