You could add a box turtle or the types
@Criscar mentioned. Either way you need to create a pen around the pond to keep them from escaping as they will leave and are quite adept at escaping enclosures. The box turtle would need land area around the pond as they aren’t semi aquatic turtles like the painteds and red ear sliders. Also, if you get a hatchling they are very prone to predation by just about everything except your fish. Even adults can get eaten by birds of prey.
Shot of one of the dark skinned dbts I keep
I keep diamondback terrapins in my ponds year round except fir the ornate dbt it comes inside for the winter. The rest Brumate in the ponds, under logs and large rocks, some even hang out in the net in the skimmer for the winter. I designed my ponds with them in mind though so they would have plenty of spots to winter over.