What’s best for swordtails?

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Neale Monks gives his advice to a reader who would like to keep some Swordtails.

Q) I’d like to keep some Swordtails in a 90x38x38cm tank. I’m thinking of about six — what ratio of males to females would you suggest? How best should I set the tank up? My water is moderately hard and the pH is alkaline at 7.8.

I’ll be having an internal power filter as I’m told they like a good flow. Any advice would be very welcome.

ROB HAND, VIA EMAIL

A) NEALE MONKS REPLIES: The length of your tank should be fine for these active, fast-swimming fish. As their streamlined shape suggests, wild Swordtails inhabit flowing streams, darting in and out of the current to snap up small prey. Of course like most other livebearers they’re as much herbivores as micro-predators, and in fact most of their food will be plant-based, particularly algae that they peck off submerged stones and wood. Nonetheless, if you imagine the sorts of brooks and streams where you’d find Minnows and Sticklebacks in Britain, wild Swordtails will be occupying the same sort of habitats in Mexico.

A water turnover rate of around 8–10 times the volume of the tank per hour should be ample. What you’re aiming for is to see your Swordtails ‘treading water’ in the current comfortably, in which case you’ll see the males sparing with each other periodically, or flirting with the females. Plants that tolerate brisk currents, such as Vallisneria, can be planted around the edges of the tank to provide shade and shelter. This is especially important if you want to breed them, Swordtails being very prone to eating any newborn fry they encounter.

Otherwise, decorate the tank with waterworn cobblestones and lime-free sand for an authentic look, adding a few smooth pieces of bogwood here and there if you need to create some hiding places for catfish or loaches. Unfortunately, few Central American catfish are traded, but a South American species like the Bristlenose will be happy enough in the conditions your Swordtails appreciate.

The water for Swordtails should be moderately hard to hard, so it doesn’t sound like you’ll need to adjust water chemistry. The one thing worth mentioning though is that Swordtails tend to struggle a bit in ‘old’ water, so frequent water changes are very helpful. In neglected tanks you will see specimens become stunted or prone to skin infections of various types. Six is a good number to start with, and in the tank you have, the ideal is probably a single male alongside five females. Males can be extremely aggressive towards each other, so where more than one male is kept you need to keep an eye out for signs of physical damage. As the fish breed you’ll soon have plenty of juvenile males, and these may be chased by the resident adult ‘alpha’ male, but being so numerous, he shouldn’t be able to do any serious harm.

As with other livebearers, females can produce more than one batch of fry per mating, so if you buy your fish from a tank containing just a single variety rather than an assortment of colour forms you will hopefully only get that variety produced in your aquarium. Wild-type, and even wild-caught, Swordtails are out there. Along with the classic Common or Green swordtail, Xiphophorus hellerii, we all know and love there’s the similar-sized Blue swordtail, Xiphophorus alvarezi, and the much smaller Yellow swordtail, Xiphophorus clemenciae, both of which may be kept in the same way as the common Swordtail. Xiphophorus species hybridise readily, so it’s best to keep a single species per tank.