Swordtails are lively livebearers from Central America, named for the long blade extending from the lower tail of mature males.
Closely related to platies, they are larger, more active, and need more swimming room than their compact cousins.
Hardy and adaptable, they come in red, green, pineapple, and tuxedo varieties.
They are strong swimmers and accomplished jumpers, so they suit keepers who provide length, a secure lid, and stable, slightly hard water.
Housing
Their size and energy call for a tank of at least 95 litres (25 gallons) with good length, kept at 22-28C (72-82F) in hard, alkaline water and filtered to keep nitrate low.
Open swimming space matters more than height for these powerful swimmers.
A tight-fitting lid is essential, as swordtails are notorious jumpers.
Provide planting at the sides and back for cover and fry refuge while leaving the central water column clear for their constant activity.
Diet
Swordtails are omnivores that do best on a varied, vegetable-inclusive diet.
Use a quality flake or pellet base, add spirulina, algae, and blanched greens, and offer frozen or live foods such as bloodworms, daphnia, and brine shrimp several times a week.
Feed modest amounts once or twice daily, only what is consumed quickly.
Like other livebearers they tend to overeat and can become constipated on a low-fibre diet, so keep vegetable matter a regular part of the menu.
Health
Swordtails are robust given clean, hard, stable water, but suffer ich, fin rot, and constipation under neglect or poor diet.
Steady warmth, strong filtration, and weekly water changes prevent most disease in this hardy species.
Mass-bred stock can be genetically weaker, so source carefully and quarantine new fish.
Watch for clamped fins, white spots, or a swollen belly with stringy faeces, signs of parasite or dietary trouble that respond to corrected care.
Temperament
Swordtails are active and largely peaceful, but mature males can be territorial toward each other, sparring over space and females.
Keeping a single male, or several with plenty of room and a female-biased group, reduces conflict and stress.
They are confident, outgoing community fish that occupy open water and rarely hide.
As ready breeders, mixed groups produce frequent fry, which adults will eat unless dense planting offers the young somewhere to shelter.
A good fit for
Larger community aquariums with swimming space
Beginners wanting an active, hardy fish
Hard-water planted setups
Keepers who can provide a secure lid
Common mistakes to avoid
Undersized tanks restricting active swimmers
Multiple males fighting in tight quarters
Open tanks letting these jumpers escape
Low-fibre diets causing constipation
Is your enclosure big enough?
Minimum volume95 litres (25 US gallons)
CompanyCannot be kept alone. The minimum is 3, 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.
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Intermediate
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