The harlequin rasbora is a peaceful shoaling fish from the soft, acidic forest streams and peat swamps of Southeast Asia.
Its warm copper-pink body and black triangular patch make a dense shoal an attractive sight in a planted community tank.
Hardy, long-lived and captive-bred, it is a good beginner fish that adapts well once a tank is cycled.
It rewards patience with years of reliable colour and calm behaviour in the right soft-water setup.
Housing
Keep a shoal of eight to ten or more in 60 litres (15 gallons) or larger, filtered and heated to 23-27C.
They look best against dark substrate in a planted, dimly lit tank with driftwood and light tannin staining that echoes their native swamps.
They prefer soft, slightly acidic water but adapt to moderately hard supplies if kept stable.
Provide gentle flow, open midwater swimming space and planted cover so the shoal feels secure enough to display full colour.
Diet
Harlequins are easy omnivores with small mouths.
A quality micro-pellet or flake forms a dependable staple, rotated with frozen or live daphnia, baby brine shrimp, cyclops and finely chopped bloodworm for variety and condition.
Feed small amounts once or twice daily, only what is consumed within a minute or two.
Varied feeding supports their colour and the general health that helps them reach their long lifespan.
Health
Harlequins are robust and disease-resistant, with most problems traceable to chilling, poor water quality or stress rather than weak stock.
Ich and fin rot are the usual concerns and respond quickly to corrected conditions and prompt treatment.
Stable warmth, soft water and weekly partial changes prevent the majority of issues.
Quarantine new arrivals, acclimatise gently, and keep them in a settled, well-cycled tank, where they prove trouble-free over many years.
Temperament
Peaceful, social and tightly shoaling when kept in good numbers, harlequins glide calmly through midwater and rarely trouble their tankmates.
A larger group settles faster, colours up and swims more openly.
They are good community fish alongside other gentle soft-water species such as small tetras, corydoras, dwarf cichlids and shrimp.
Avoid large, boisterous or predatory tankmates that intimidate the shoal or keep these mild fish hiding among the plants.
A good fit for
Beginners wanting a peaceful, long-lived shoal
Planted soft-water community aquariums
Keepers seeking calm, reliable colour
Shrimp-friendly and biotope-style tanks
Common mistakes to avoid
Keeping too few, causing chronic stress
Hard, alkaline water dulling their colour
Adding them to an immature, uncycled tank
Housing with large or aggressive tankmates
Is your enclosure big enough?
Minimum volume60 litres (16 US gallons)
CompanyCannot be kept alone. The minimum is 8, and the space above is for that group.
In a fish tank the patient is usually the water. Test before you treat — dosing a healthy tank for a disease it does not have makes things worse.
Go now
Any detectable ammonia or nitrite in a stocked tankLarge water change now with dechlorinated, temperature-matched water. Stop feeding. Test again in 12 hours. Both are directly toxic. How toxic the ammonia reading is depends on pH and temperature, so the same number can be harmless or lethal.
Now
Fish gasping at the surface, or crowding the filter outflowIncrease surface agitation immediately, then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems.
Now
Sudden deaths, more than one fish in a dayTest ammonia, nitrite, nitrate and temperature before doing anything else. Do not add medication yet. Multiple simultaneous deaths point at the water or the equipment, not at a pathogen.
Now
Today
Flashing — repeatedly scraping against decor or substrateTest the water, then look closely for parasites. Irritation is either chemical or parasitic; the test tells you which before you medicate.
Today
White salt-grain spots over the body and finsBegin the appropriate treatment today and check the temperature. The parasite's life cycle is temperature-driven, and it multiplies with every cycle you wait.
Today
Soon
Clamped fins, hiding, or refusing food for more than two daysTest the water and review recent changes. Usually the earliest visible sign of a parameter drifting.
A few days
Never add a new fish to a tank that is not fully cycled, and never treat blind. A test kit is cheaper than the fish.
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.
Intermediate
Beginner
Beginner
Beginner
Beginner
Beginner