The Clarkii, or Clark's anemonefish, is the hardy middleweight of the clownfish family, with a dark brown-to-black body, two broad white bars, and bright yellow fins and tail.
It has the widest natural range of any clownfish, from the Persian Gulf across the Indo-Pacific to southern Japan, and that adaptability shows in the aquarium.
Females grow noticeably larger than males and can reach about 15 cm (6 in), a full size class above the Ocellaris.
Its defining trait is anemone partnership: the Clarkii is the only clownfish recorded hosting in all ten natural host anemone species, so it is the species most likely to actually use an anemone in captivity.
It does not need an anemone to thrive, and anemones themselves are demanding animals best left to experienced keepers with a mature system.
Colours often darken with age, and captive-bred stock is widely available and worth insisting on.
Choose a Clarkii as a step up from the beginner clownfish, at least as hardy as an Ocellaris but larger, more active, and more territorial.
It is still a decade-plus commitment in a fully equipped marine system.
Housing
A bonded pair needs a cycled marine tank of at least 150 L (40 gal), and more is better, because this is a larger, faster swimmer than the Ocellaris that patrols open water rather than hugging one spot.
The old 30 gal (115 L) figure is a bare minimum for a single fish; give a pair the extra length and swimming room.
Provide live rock for territory and cover, plenty of open water, and a tight-fitting lid, since clownfish are determined jumpers.
Maintain salinity 1.024-1.026, temperature 24-27°C (75-80°F), pH 8.1-8.4, ammonia and nitrite at zero, and nitrate below 20 ppm.
Add fish only to a fully cycled system that has finished processing ammonia and nitrite.
If you want the classic clownfish-in-anemone picture, a hardy bubble-tip anemone (Entacmaea quadricolor) is the usual partner, but it demands strong reef lighting, stable water chemistry, and a tank matured for six months or more.
Treat the anemone as a separate advanced project, not a Clarkii accessory.
Diet
Clarkii are unfussy omnivores with big appetites for their size.
Feed a quality marine pellet as the staple, plus frozen mysis, brine shrimp, and finely chopped seafood, with algae- or spirulina-based food for balance.
Offer small amounts once or twice daily, only what is eaten within a couple of minutes.
They feed boldly and can outcompete shy tankmates, so make sure slower fish actually get their share.
Captive-bred fish usually take prepared foods immediately.
Vary the diet to keep colour strong, and remember that overfeeding is the fastest way to wreck marine water quality.
Health
The Clarkii is arguably the toughest clownfish, but it shares the family's weaknesses: marine ich (Cryptocaryon), the fast-moving clownfish disease Brooklynella, and the even deadlier marine velvet (Amyloodinium).
All three usually follow stress, poor water quality, or skipped quarantine.
Quarantine every new arrival for several weeks in a separate tank before it joins the display.
Watch for white spots, a greasy or peeling skin coat, scratching against rock, clamped fins, or rapid breathing, and act the day you see them.
Stable salinity and temperature prevent most problems in this robust species.
A healthy captive-bred Clarkii routinely lives 10-15 years, and sometimes longer, so plan for the commitment, not just the purchase.
Temperament
The Clarkii sits between the other clownfish in temperament, bolder and pushier than an Ocellaris but well short of a Maroon's outright aggression.
Juveniles are peaceful, while a mature female becomes territorial, chases fish that stray into her patch, and may nip the hand that reaches in during maintenance.
Like all clownfish they start life male, and the dominant fish of a pair changes sex to become the larger female.
Keep one fish or one bonded pair, and never mix clownfish species in a single tank.
They are reef-safe with corals, though a fish without an anemone may pester a coral by adopting it as a substitute host.
Choose tankmates that can hold their own around a confident, fast-moving fish.
A good fit for
Keepers stepping up from their first marine or reef setup
Reef tanks that may add a hardy host anemone later
Aquarists wanting the hardiest, most forgiving clownfish
Tanks with open swimming room for an active, open-water fish
Single-specimen or one bonded male-female pair setups
Buyers who insist on captive-bred stock
Common mistakes to avoid
Buying a host anemone before the tank is mature enough to keep one alive
Mixing Clarkii with other clownfish species in one tank
Underestimating a mature female's territorial temper at maintenance time
An Ocellaris-sized tank for this larger, faster, more active fish
No tight lid - clownfish are determined jumpers
Skipping quarantine, which invites Brooklynella, velvet, and marine ich
Is your enclosure big enough?
Minimum volume150 litres (40 US gallons)
CompanyFine alone or with company. Solitary or one bonded pair only
In a reef, stability matters more than any single target number. A parameter moving fast does more damage than a parameter sitting slightly off.
Go now
Any detectable ammonia or nitriteWater change now with matched, mixed and aerated saltwater. Stop feeding. Directly toxic, and a marine system has less margin than a freshwater one.
Now
Temperature or salinity has moved sharplyCorrect slowly, over hours, not minutes. Check the heater and the top-off. The speed of the change does the damage. Correcting fast repeats the injury in the other direction.
Now
Rapid gilling, or a fish hanging in the flow breathing hardIncrease aeration and check temperature and salinity. Warm water holds less oxygen, and a heater failure shows up here first.
Now
Today
White spots, or a fine dusting like icing sugarIdentify which parasite before treating. Most reef tanks cannot be treated in place. The two common parasites look similar and need different handling; velvet can kill a tank in days.
Today
Corals closed for more than a day, or receding tissueTest alkalinity first, and compare it against the last reading rather than a target. Unstable alkalinity is the most commonly cited coral killer, and a swing shows up in the trend, never in a single point.
Today
A new fish added without quarantine now looks unwellRemove it to a separate system today if you can. Nearly every wipeout in a reef tank traces back to a skipped quarantine.
Today
Quarantine every new animal, including corals and inverts. It is the single practice that separates a reef that lasts from a reef that restarts.
This is general guidance for a healthy adult, not a diagnosis. An exotics vet who knows your animal always overrides it — and the time to find one is before you need one.
What it really costs
Setup, monthly, first year and across a whole life, itemised and with the electricity worked out from the actual wattage. Published figures for the same animal disagree by a factor of four, so the calculator shows ranges rather than picking one.
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