There are about 1,000 species in 10-13 families within this order of small, mostly freshwater fish, found in warm regions across the globe. This group includes familiar favorites like killifish, guppies, mollies, and swordtails.

Most Cyprinodontiformes prefer shallow waters—ponds, streams, marshes, and even temporary puddles. Some species can thrive in extreme conditions such as salty water, oxygen-poor environments, or habitats that dry up seasonally. For example, certain killifish lay eggs that can remain dormant in dry mud until the rains return.

These fish are generally small, often just a few inches long, but many males dazzle with bright colors and striking patterns. Their colorful appearance and lively behavior make them popular choices for home aquariums.

Despite their diversity, Cyprinodontiformes share some common traits:

• A single dorsal (top) fin positioned far back on the body.
• Often upturned mouths, perfect for feeding at the water’s surface.
• Diets of insects, tiny crustaceans, and plant material.

Some species in this order lay eggs, while others give birth to live young. The livebearers—like guppies and mollies—are especially popular among people with aquariums because they’re easy to breed and raise.