Swordtail: why two males will not share a tank, and how to set the tank up
The swordtail (Xiphophorus hellerii) is sold as a beginner's fish, and in terms of hardiness it is one: it tolerates a very wide temperature range and breeds on its own. Where it disappoints is behaviour. A badly composed swordtail tank turns within weeks into a scene of permanent chasing — and that is not down to individual temperament, it is structural. The swordtail species profile covers its basic requirements.
Males form a hierarchy, including in the wild
This is not an artefact of captivity. D. Franck and A. Ribowski observed swordtails in their native range, in the creeks of Veracruz in Mexico, and published their findings in 1993 in the Journal of Fish Biology: males form dominance hierarchies in shallow areas and spend a substantial share of their time defending their rank, averaging 11.1 chases per five-minute period (study on Google Scholar). Eleven chases in five minutes, in a creek where a subordinate can flee for tens of metres. Inside 80 centimetres of glass, it has nowhere to go. That is the whole difference between a natural behaviour and a harmful one.
Females pay a price too
Sexual harassment is a measured phenomenon in poeciliids. Marco Dadda compared six species in a study published in 2015 in Frontiers in Psychology: in the swordtail, around 30% of male sexual acts are forced copulations, and the higher that proportion is in a species, the more closely females associate with one another in the presence of a male — a social counter-strategy (study on Google Scholar). The practical consequence: a lone female with a male is a female under continuous pressure, which feeds less well and hides more. The minimum sensible sex ratio is two to three females per male, with a single adult male in a normally sized tank.
Tank shape matters as much as volume
This is the most directly usable finding, and it is counter-intuitive. A team led by K. Magellan showed, in a study published in 2012 in the Journal of Applied Ichthyology, that at identical volume a taller tank with a smaller footprint significantly reduces attacks by the dominant male on subordinates, without changing the frequency of courtship displays (study on Google Scholar). The simplest reading: what the male defends is floor area, not water volume. Breaking that floor area up with roots, rocks and dense planting effectively multiplies the number of available territories. Our planting benchmarks are in easy plants without CO2.
And what about the sword
The male's caudal sword is not a free ornament. Alexandra Basolo showed, in a study that became a classic published in 1990 in Science, that female platies — a species with no sword of its own — prefer males given an artificial sword: female preference therefore predates the appearance of the sword in the group's evolution (study on Google Scholar). In other words the sword is a powerful sexual signal, and a male with a fine sword will be all the more exposed to challenge from other males. It is not an aesthetically neutral criterion when you put a group together.
The setup that lasts
In practice: a tank of at least 120 centimetres frontage for a group, generous length but a fragmented floor area, filtration turning over four to five times the volume per hour, and a single adult male with three or more females. Above all, plan for what comes next: like all livebearers, the swordtail breeds whether you ask it to or not, and males from those broods will start challenging the dominant at around four to six months. Population management is the group's real difficulty, as our article on population explosions in livebearers explains. To compare with more peaceful livebearers, see molly or platy, and the hardware in aquarium and habitat.
Frequently asked questions
Can two male swordtails be kept together?
Only in a very large, heavily planted tank, and with no guarantee. In a normal volume the dominant will harass the subordinate to exhaustion.
What should I do with young males from the broods?
Rehome them before sexual maturity, at around four months. Waiting for a conflict to break out means managing a problem that is already entrenched.
Is a tall tank really better than a long one?
At equal volume, the 2012 study shows fewer attacks when floor area shrinks. A long tank remains excellent if it houses only one male and the substrate is visually broken up.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.