Guppies, Swordtails, Mollies & Platies


This group is known as livebearers because unusually for fish the females give birth to free-swimming live fry (ovoviviparous) rather than laying eggs. In the wild, these species are found in Northern, Central and Southern America. Almost all aquarium specimens are captive bred.

Water Requirements

This group of fish are relatively hardy and will adapt to most aquarium water. However, it is advisable to keep the water parameters within the following guidelines although fish may acclimatise to different water conditions over time.
Temperature: 18 to 28C (64 to 82F).
pH: 7.0 to 8.0
Ammonia: 0mg/l (0.02mg/l may be tolerated for short periods)
Nitrite: 0mg/l (0.2mg/l may be tolerated for short periods)
Hardness: Slightly hard to hard (9 to 19dH)


Guppies, mollies and swordtails can be kept in brackish (low salinity) water, however they are best kept in freshwater.
These fish can thrive in a home aquarium in the which the water quality is well maintained and live for 3 to 5 years. They belong to the family Poeciliidae.
Males in all of these species can be identified by their elongated anal fin, known as the gonopodium. Females have a triangular anal fin.
Male swordtails have an extension to the caudal (tail) fin.
Males guppies are often brightly coloured compared to the females. This difference has been selectively bred as the wild caught species are much duller.
Maximum body length:
Platy: female 6 to 10cm and male 4 to 8cm
Molly: female 10 to 16cm and male 10 to 15cm
Swordtail: female 10 to 12cm and male 8 and 12cm
Guppy: female 4 to 6cm and male 3.5 and 5cm

Aquarium Requirements:

All of these species prefer a planted aquarium for two reasons:
1. They are all omnivores and will eat some of the plants
2. If breeding, the fry will have plenty of cover A heater, filter and aeration are also required, and it is advisable to
have lights, both to aid the growth of plants and to simulate daylight to bring out natural behaviours and colour of the specimens. The aquarium should be large enough to comfortably house at least three fish. A ratio of 2:1 (female: male) is best as this helps to avoid single females being pestered by numerous males.


At least once every two weeks, a partial water change of 25 to 30% is strongly recommended (a siphon device is useful to remove waste from the gravel). The water should be tested regularly to ensure that pollutants such as ammonia and nitrites donít build up. Ensure that you either allow the replacement water to stand or aerate it to
remove any chlorine present. Ideally, treat all replacement water with tap water conditioner before adding to the aquarium.
Filters should be checked for clogging and blockages. If the filter needs cleaning then do not run it under the tap as any chlorine present may kill the beneficial bacterial population that has established in the media. Instead, it can be rinsed in the tank water which is removed during a partial water change as this reduces the amount of bacteria which are lost.
Good husbandry is essential as these fish can be stressed by even the smallest amounts of ammonia and nitrite. Test the water to monitor the ammonia, nitrite and nitrate levels every week, especially during initial set-up and after adding extra fish.
If only mollies are kept then the addition of aquarium salts is advisable, this is not required with swordtails, guppies or platies.
Check with your OATA retailer as to how much salt should be added to your aquarium. Mollies are capable of tolerating high salinities however the increase of salinity should be gradual to avoid shocking the fish.


This group of fish are omnivores, however, a vegetable-based diet is often preferred.
The diet for your fish should match this, including flake, frozen, spirulina and freeze-dried foods.
The fish should be fed what they can eat in a few minutes 1 to 2 times a day. Remove any uneaten food to reduce waste build up.

Potential Problems

A water quality problem will affect fish behaviour and can be shown by clamped fins, reduced feeding, erratic swimming and gasping at the surface. Immediately test the water if any of these symptoms are shown. If in doubt ask your OATA retailer for advice.


These fish make good community tank inhabitants. Generally, they are not aggressive, although male Sailfin mollies (which can be somewhat larger than ordinary mollies) can become aggressive if not paired with a female.
These fish will do well with other passive fish that have a preference for medium-hard water conditions.
If wishing to breed from your fish then avoid stocking larger fish species which could easily eat the fry as they are born.


All of these fish are easily bred in home aquariums, in fact it is probably difficult to stop them breeding if you have both sexes.
It should be noted that the females may produce between 20 to 80 fry following a pregnancy (which may last 24 to 30 days). It is advisable to keep the female in a suitably large breeding net.
She should be separated from the newly arrived young to avoid her eating them. The fry should be reared separately until they are large enough to feed freely and not be easily eaten themselves.
Try to avoid stressing a pregnant female.


Before purchase make sure that:
You have the appropriate equipment and position for the aquarium.
You have researched all the species you are interested in and your final choices are all compatible.
You are familiar with how to transport and release your fish.
You are aware of the daily, weekly and monthly maintenance your aquarium will require.
You are prepared to look after your fish properly for the duration of their life.


Glass or plastic aquarium
Gravel cleaner
Water testing kit
Tap water conditioner
Heater & thermometer
Breeding net (if appropriate)
Marine salt (if keeping mollies in brackish water)
Before purchase make sure:
The aquarium is of a suitable size
Water parameters are as advised in this leaflet
If adding to an existing set-up ensure these fish are compatible before introducing.

Important things to remember

Always buy…

test kits and regularly check the water for ammonia, nitrite, nitrate and pH. This will allow you to make sure the water in your aquarium is not causing welfare problems for your fish.
Establish a routine…
for testing the water in your aquarium. Record your results to enable you to highlight fluctuations quickly. Also check the temperature of the water.


the water in the aquarium within the accepted parameters highlighted in this leaflet. You may need to do regular water changes to achieve this.

Always wash your hands…

making sure to rinse off all soap residues, before putting them into your aquarium. Wash your hands again afterwards and certainly before eating, drinking or smoking.

Never siphon by mouth…

A fish tank can harbour bacteria which can be harmful if swallowed. Buy a specially designed aquarium gravel cleaner which can be started without the need to place the siphon in your mouth.

Credit to OATA

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