Sounds like you have a pregnant female platy. When you see the black dots, is it on the outside on the scales or does it look like the spots are "inside" the belly? If they are inside, then what you are actually seeing is the eyeballs of the baby fish inside mommy's belly. Quite often, people think they are buying male fish, and livebearers like platys, mollies and swordtails can be tricky unless you know what to look for. Sometimes these fish are already pregnant when you buy them. To encourage her to give birth, put some floating plants in the tank, raise the tank heater temp two or three degrees and offer a meaty meal like frozen brine shrimp. Sometimes just having a thunderstorm come through your area is enough to get fish to spawn of give birth. Something about the barometric pressure. Check out www.aquariumfish.net to get more answers on fish. Congratulations!
