Baby Turtle is sick! please help?

Okay, so I got a baby turtle at the beach on spring break. He was happy and healthy in the big 10 gallon tank I put him in. I bought Simon because I felt bad for him.. in a crowded tank at the beach. I got him and a few weeks ago he stopped eating and his shell is turning brown. He used to swim around alot and now he won't. We bought all kinds of food- pellets, dried fruit (made for turtles), and dried shrimp. He just sits there. He's a red eared slider. Pleasse help, I bought something to put in his tank to keep his shell get better, but it didnt wokr. I want him to get better! Please help :(