Is she pregnant?
Look down over the top of the fish and see if any scales are sticking out! If they are she IIis not pregnant, but bloated. She is a livebearer, she will drop fry about every 28 days +/-, and she can drop 3 to 103 each time. She can also self-fertilize and only needs that one contact with a male to be pregnant over the next several months. Near the end of the pregnancy, she will become boxy shaped \_/ , hide, and get cranky. Moving her to another tank at this point (late in the pregnancy) would probably stress her out.
courtesy of Alice Chaos
Moving a pregnant fish to another tank
How big is the extra tank and is it cycled? Keep an eye on the water parameters in the tank. Only feed her what she can eat in under two minutes. Keep the tank cycled and clean. Add some floating plants - these will help the fry hide. Hornwort is great and has some natural nutritive value. Wisteria is also good. Once the mother is done, put her back into the main tank. For several days, you will only see 2 eyeballs on the mostly invisible bodies of the fry. In addition to the hornwort, the fry can also eat fish flakes that have been powdered. Tiny bits of the yolk from a hard boiled egg works too. There is also fry food available in stores. Use a turkey baster to clean any tank debris. Work diligently on that tank water. Once the fry are too large to eat, they can moved into the main tank.
Make sure you have a backup plan for your future population explosion! :) Fry are adorable!
courtesy of Miss_Teryland
Leaving a pregnant livebearer in the main tank
If the female remains in the main tank, she will have to deal with the male. She can still have the fry in the main tank. The fry can hide in the floating plants, cracks and crevices. To increase fry survival rates, catch them in a small cup, and put them into a floating breeder box. Do not put the mother in the floating box! Again, make sure you have a plan for keeping all of these fish - don’t let your tank get overstocked!
courtesy of cskk