My Red Eared Slider is Swimming Lopsided?

I know it is one of the symptoms of RI, but he still eats normally, is active, and basks regularly. My RES doesnt wheeze or gasp--its just the lopsided swimming. It's not very pronounced, but he tilts just a little to the left...should I take him to the vet? I'm not sure how old he is. I've had him for about a year, and he is 2 and a half inches CL. I have him in a 10 gallon with a heat lamp and dry basking spot, and he goes out for an hour each day for natural sun and UVB. I don't have a heater, but the temp stays at a constant 25 Celsius. (Oh, and I don't know for sure if my RES is a boy, since he isn't old enough, but I'm pretty sure "he" is. And if i don't need to take him to the vet, what should I do for him?