Greebo is right, longicollis are a coldwater turt, although they tolerate higher temps. mine are at room temp 25c now and around 12c-15c in winter and still active though feeding less. Further I never let it rise above 25c and some would say thats too high, I use frozen water bottles to keep the temp down in warmer weather. I might just add that the higher the temp the fasterthe metabolism, much the same as snakes/lizards.
Constant higher temps are not good. Cb turtles commonly kept at higher temps, (and fed too much) grow too quickly, this can lead to long term problems.
What species Ashley?