Live-bearing freshwater fish can be a fun addition to your home aquarium. If you buy fish from pet stores that house opposite genders in the same tank, you have a good chance of buying a pregnant fish, particularly if you select platys, swordtails, mollies or guppies. If you want to breed freshwater fish on your own, select both males and females from the same species and add them to your tank.
A primary telltale sign of a pregnant live-bearing freshwater fish is that her abdomen begins to swell. You may see a dark or red triangle forming on her underbelly as the baby fish, called fry, grow and develop. While it varies by breed, most freshwater fish are pregnant for 20 to 40 days, during which time you will start to see tiny eyes visible through the mother's abdomen.
As a mother reaches her time to give birth, she may become slow, lethargic and unwilling to eat as much as usual. You may also find her hiding, particularly if you have an aquarium with leafy foliage. Live-bearing freshwater fish prefer to give birth in or around this type of material so the fry have some place to hide and stay safe from predators.
Assisting Your Fish
If you want to ensure your young fry stay alive and aren't made into tiny meals for other fish in your aquarium, you'll want to move the mother into a breeding tank, also called a birthing tank, before she starts to give birth. A birthing tank is typically a small plastic container that you hang along the top inside of your aquarium. The mother may act a bit skittish at first, because there’s nowhere to seclude herself, and will usually settle herself into a corner.
Caring for Fry
A birthing tank is designed with a V-shaped slotted bottom that the baby fish drop through once they're born. This keeps the mother from eating them. Feed newborn fry right away. While they are fully developed and can eat regular flaked fish food, you’ll want to keep them in their separate mini tank and let them grow large enough that they can survive in the main tank before setting them free. How long this takes depends on the size of the other fish you have in your tank community.