Painted Turtle growth

Although I have had success with tortoises, I am new to the world of aquatic turtles. I recently acquired a group of four Southern Painted Turtle hatchlings.

I am looking for advice from anyone working with these or another similar turtle. I feel confident that their setup is correct, but I am concerned regarding their growth. The two larger turtles (none of the four are over 3") are beginning to show signs of their shells turning up at the edges. Is this normal or do I have a nutritional problem involved here.

Any and all responses are very much appreciated.

Thank you,

Mike Greathouse
Reptilian Dreams