The fish don't get black spots around the gills.
There gravid spot will be black.
Get you some info.
You say you have just bought the platys was it kept with snails.
http
/www.fishforums.net/content/Common-L...nancy-And-More/
Black Spot
Symptoms:
Your fish will have small black spots measuring about 2 millimeters in diameter. These can be found anywhere on the fish’s body and fins. Cysts may be found in the internal organs. It is typical for African Cichlids, however, to constantly have little black spots, especially around their mouths. These black spots should not be mistaken for "Black Spot." These are scabs, which have resulted from digging in the gravel or from fighting.
Cause:
Larvae of parasitic digenetic flukes, Cercaria and Metacercaria. The spots seen contain larval stages of digenetic flukes. The life cycle of these parasitic flukes begins when fish-eating birds and other animals ingest fish infected with the parasites. Once ingested the parasites mature in the intestines of the host animal where they produce eggs. The eggs are then deposited into the water where they hatch and infect the livers of aquatic snails. The parasites then develop into a second and then third larval stage before leaving the snail to seek out a fish host. This type of parasitic infestation is most common in wild caught fish and pond fish.
Treatment:
Remove all snails from the aquarium or pond to end the cycle of infection. Treatment is rarely necessary and can be difficult if not impossible. Minor infestations may clear without treatment as the larvae will eventually die and be absorbed by the host's immune responses. Copper Sulfate may be helpful in severe cases.
What type of platys is it, just to make sure its not markings.
