Livebearers, like their name suggests, retain their eggs in their body and give birth to free-swimming fry. The guppy, molly, platy and swordtail are popular livebearers. These fish are hardy, colorful and very easy to breed in captivity.
Livebearers are schooling fish. This means that they feel most comfortable in groups. In an aquarium, keep them in groups of at least five per species. Additionally, males can act aggressively toward females, so keep several females per males. In most common livebearers, these guidelines work well. However, a few more rare species, like the halfbeaks and pike topminnows are very predatory and best kept alone or in groups of similarly-sized fish.
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Livebearers' tolerance of a wide variety of conditions is part of what makes them popular. As a group, they are generally tolerant of most freshwater aquarium conditions. Ideally, livebearers prefer an addition of 1.25 tablespoons of aquarium salt (rock salt or kosher salt works too) per gallon, and a temperature of 75 to 80 degrees. However, mollies are an exception. They are more delicate than other livebearers and need a water temperature of at least 82 degrees. You can achieve this with a regular aquarium heater. In their case, aquarium salt is not optional, and they can even tolerate seawater.
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The common livebearers are considered community fish. This means that they stay small, have a peaceful disposition and require very little special care. They get along with any fish that does not prey on them or harass them. Again, there are exceptions in some rarely-kept members of the family. Also, mollies' love of salt and higher temperatures may limit their tankmates. But as a group, they get along well with other small peaceful fish.
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The defining characteristic of this group of fish is that they give live birth regularly. A single mating can produce a dozen batches of offspring. However, breeding them is not foolproof. Most livebearer parents are cannibalistic. To reduce predation on the young, you can either move pregnant females to another tank to breed or densely plant the main tank to give the baby fish hiding places.