What type of turtle?
In general do not add anything to a turtle tank that you do not want to end up as turtle food eventually.
The safest options are fast fish like long-bodied (pond) goldfish, cichlids, and few carefully selected others. There are many things that will effect compatibility though, like: tank size, turtle species, turtle size, how it was raised (with fish?), what it has been fed (live vs. pellets), filtration, decorations, water change schedule, and individual personalities.