RES are pretty tolerant to most water conditions. Don't worry about ph/kh/gh/fe etc. Some people recommend a slightly acidic tank, since it can help keep some kinds of fungus under control, but if you keep a clean tank, you don't really need to worry about it.
Ammonia, like with fish, you want to keep on the lower side while the tank is cycling. Same with nitrite. It's ok if nitrate creeps up a little bit once and a while, but make sure you keep it under control. My nitrate tests go up to I think 110 ppm. Usually when it hits 110, I do a water change, or sooner of course.
If you keep fish with them, I probably still wouldn't worry about it too much, since fish you keep with your turtles are likely going to become food long before they'd have any ill effects from improper water chemistry
