What is wrong with my Diamondback Terrapin Turtle?

I have had two turtles for years and they have been in the same tank 120 gallon. A snapping turtle and a diamondback terrapin. They grew up together and finally had to move the snapper to another tank because he would always go after the diamondback. I would always try and keep fish in there so he wouldn't get hungry but when I would feed them pellets I would have to sit and watch the entire time just incase. The diamondback finally got to the point he would sit on the log and not want in the water so I moved the snapper. Its been a month and hes acting strange. I don't think hes sick when I feed him hes active but usually he swims around and is active all the time. I am hoping hes not missing the snapping turtle!? Do I need to get him another turtle in there? I have two other turtles but they are alot smaller then him and they have been other in the same tank for awhile and don't want to mess them up? The Diamondback is about 6 inch and the other two baby turtles are less then 1/2 that size. Any thoughts or input would be great thanks!