Red eared slider questions...?

We had a roommate move away and leave a red eared slider with us. I want to take good care of him but am clueless...He was found at about nickel or quarter size right after hurricane Katrina, so he's going on 3. He's now about the size of the palm of my hand. He's been fed small multicolored bones about 1 inch long, that resemble dog treats. I'm not really sure what the heck they are because they are in an unmarked bag. I read they were supposed to have fruit, and all kinds of other things, but he wouldn't eat any of it with several days trying him. I figured he might do better on turtle pellets, since the treats are about the same consistency as pellets, but I don't know what kind to get or how much to feed him. He has plenty of flat logs and rocks to climb in, but how many inches of water should be in his tank? What about a filter? Our old roommate had the turtle in his room, and when I found the conditions of this creature I was appalled.