Two of the most popular aquarium fish, the Swordtails and Platies, both come from the Xiphophorus family, sharing similar needs and behaviours in the aquarium. Both fish are relatively small, hardy, peaceful, and are available in a huge variety of colour forms, making them an ideal aquarium fish.

Species

There are several species of Swordtail, with Xiphophorus helleri being the usual one found for sale in the trade, although crosses can occur often. There are two species of platy commonly found for sale, Xiphophorus maculatus, the common Platy, and Xiphophorus variatus, the Variatus Platy.

Of these species, both Platy and Swordtail there are a myriad of line-bred colour forms, in virtually any colour and pattern variation. As has been seen before with such intensive breeding to produce and maintain these colour forms at high production rates, there is a trade-off and the fish for sale nowadays are much less hardy than those seen 10 years ago. The problem is not as severe as with other fish such as Guppies or Dwarf Gouramies however, and both Platies and Swordtails are still relatively hardy and suitable as a newcomer's fish.

Natural habitat and diet

All of the Platy and Swordtail species originate from Central America and Mexico habitats and are normally found in smaller rivers and streams where there are fewer large predators. Moving waters seem to be preferred and the fish are often found in pools along fast flowing waterways. In these areas there is plenty of plant and algae growth from the sunlit shallow waters, and these fish are good grazers and micro-predators, feeding on algae growth and tiny aquatic creatures.

Being natural grazers they will require some vegetable matter in their aquarium diet, this can come from naturally occurring algae, or from vegetable enriched foods. Apart from this requirement, there are no specific needs and the fish will thrive on a diet of mixed proprietary foods supplemented with small live or frozen foods.

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Sexing

Identifying sexes is easy with both these fish and can be done by observing the shape of the anal fin (the single fin situated on the underside between the tail/caudal fin and the paired pelvic fins) On females the anal fin opens into a triangular shape whilst on males the fin is pointed. The males pointed anal fin is actually modified into what is called a 'gonopodium' and is used for transferring milt into the female for internal fertilisation.

Swordtails can also be sexed by the sword extension, which gives the fish its name, and which only the male's posses. The adults will also reach different sizes, with females being the larger sex; in Platies males will reach around 3-4cm, whilst females can reach 6cm, male Swordtails will grow to 10cm, and females to 12-13cm.

Sexing can be tricky in very young fish, of which the males may not have yet developed their pointed anal fin, or male swordtails which may not have developed their 'sword'. Interestingly, female swordtails have the ability to change sex into males, which can occur if an entirely female population is kept, or if the female reproductive organs are damaged, or cease to function correctly.

Breeding

Like many other livebearers, both Platies and Swordtails breed easily and readily both in the aquarium and in the wild, producing between 20-100 young in each batch. This 'fast and many' method of breeding goes hand in hand with a lack of parental care, and the fully formed young are left to fend for themselves as soon as they are born.

Inducing breeding requires no specific conditions, although a varied diet with additional algae or vegetable based foods and a good overall environment will encourage more successful broods. You can tell if a female is carrying young as she will develop a dark patch at the rear of the belly and will become very rounded, however it is difficult to guess when she will give birth since the size of broods varies so much.

Both Platies and Swordtails have a period between mating and birth of around five weeks, and the females can store milt from one mating to produce up to five broods. In a community aquarium it is not uncommon for all, or at least most of, the young to be eaten often before they are even noticed by the fish keeper. To prevent this and retain as much of the brood as possible it is best to remove pregnant females into a separate aquarium stocked with plenty of vegetation for the young to hide amongst. If this is not possible, breeding traps can be used in the main aquarium to hatch and raise fry in.

The fry will take small foods such as small daphnia, Cyclops, and newly hatched brine shrimp as well as crushed flake, straight from birth.

Aquarium conditions

Both Platies and Swordtails are undemanding in terms of water conditions and can be kept in most community aquariums. Although not strictly a shoaling fish, they both look and cope better when kept in groups. For Platies, a 60cm+ aquarium will suffice, whilst fully-grown Swordtails will require a 90cm aquarium.

Although hard water is preferable, they will acclimate to softer conditions in well-maintained aquariums. Although the fish are not brackish species, as with many other livebearers, a little salt added to the aquarium will improve disease resistance and immunity.

Warmer water, around 25-28C (77-82F) will also improve overall health, and an area of strong flow is appreciated by Swordtails. In the wild the fish can be found in cooler water, but this should not be the case for the tank-bred varieties. There is some confusion in many retailers over whether Platies are suited to unheated aquaria, partly because the stronger strains of the past would tolerate such conditions whilst those available today will not. With today's fish stocks, the common platy X. maculatus, will not do well in cooler water, but the Variatus Platy X. variatus will do perfectly well in unheated indoor aquaria, and is found in cooler water in the wild.

Aquarium decor should contain planted areas and plenty hiding spots as well as open swimming space. Hiding spots are very important for females, which can be harassed and stressed by males to the point of ill health, especially when the females are already heavily pregnant. Plenty of little hiding spots will allow the females a retreat and place of recovery.

Tankmates

As with other livebearers, the ratio of males to females is important and should be at least two females to every male. Any retailer should be able to pick you out either all one sex or a specific ratio of males to females. In addition, when keeping Swordtails, it is important to either have just a single male or a group of four or more. If there are just two or three males, one may become dominant and bully the weaker males, whilst in larger groups the males are much more sociable. Avoiding very nippy fish, which may attack the male Swordtails sword, both Platies and Swordtails, can be mixed with almost all similar sized and smaller peaceful tank mates.