I don't recommend acidifying the water for Cyclemys but if you must you can use spagnum. Just add it into the pond itself or insert it into a canister filter chamber. You can buy liquid aquarium pH lowering chemicals, even natural extracts that are similar to the Amazon...but these might get expensive if you are treating a pond. Many times I have noticed that turtles acquire fungal outbreaks when the water pH is raised and lowered. If you put the turtle into a semi-stable pH they can acclimate, just like many fish do. Everytime you adjust the pH in your pond without pretreating a batch of water (and then adding that water), you will cause the pH to bounce before it settles. This bouncing can cause fungal outbreaks.
Goodluck!
Mike