What is wrong with fish tank.. please help....?

Recently i got a fish tank. The capacity is approximately 10 gallon. I have two platys and one molly left. I had two more platys but both died with the symptom of slight reddish near fins, some fungus like white stuffs in the tail, gasping in water, shaking, remaining still in the bottom. Their appetite however didnt reduce. Today I have changed the water, cleaned the tank thoroughly.. However i still see that that my other remaining fishes are not happy... i thought first there was fungus so i gave them fungal medicine and also i have added sea salt. I donno what else should i do. My one Platy gave birth almost 25 babies on the very next day i bought her from the store. The babies are fine in my nursery tank. They are 2 days old. if there is anything wrong with the water then i think the babies would not have survived . I donno what should i do. please help.....is it ammonia poising, nitrate poisoning, fungal disease ?? my white molly died today and she had slight reddish mark on her body and also some scalp infection and slightly bloated fin!! please help....