Painted turtles keep scratching their heads and the bottoms are tuning a little brown?

Hi I recently was given two baby painted turtles. The bottom of one turtle has turned brown and am not sure what to do for him. It would seem that maybe this is a bacterial infection?? It also share the tank with another turtle who seems to have a little brown starting on the bottom shell. What should I do. They also are consistantly scratching their heads what does this mean. thanks for your help.

Sincerely Amy Comfort