What is causing my turtles shell to turn white?

I got two red eared slider turtles from a place at the beach. (very small) The lady whom i got them from had them in a 10 gallon tank and said i should feed them every 5 days. Well i brought them home and did a little research. i found out they have to have a basking area, and need to be fed everyday. so i bought a 10 gallon tank, a place to bask, and a filter. i also use reptile condition to clean water when i change the tank about once a week. Well ive had them almost a month and the other day i noticed stuff floating from there skin. today i see white blotches, so i picked one of the turtles up and the shell is kind of soft, what wrong?