He looks healthy to me....bright eyes and an alert posture. Water turtles are by nature "baskers", swimming is for the most part a function of feeding. I agree with Yorkie that Reptomin will provide a balanced diet. You mention the temperature of the basking area, but not the light source. When basking, a UV source is important. Turtles require UVB to produce the Vitamin D they need to to incorporate calcium into their metabolism and harden bones and shell. Hopefully you have a light source that approximates sunshine. As healthy as he looks...I suspect you do. < g >
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Craig