White poop could be internal parisites. (Tapeworms, ect.) I've never had to treat for it, so I'm not sure of how to diagnose it or treat it properly. If possible a picture might help, or a detailed description of the "poop".
You mentioned you have salt in the tank: How much? How often do you add it? is it table salt, aquarium salt, rock salt, ect? If it is table salt, look at the ingredients. I've read that Yellow Prussiate of Soda (it's a cyanide compound)which is used as an anti-caking agent in some table salt is bad for fish. Silicone dioxide, another anti-caking agent used in salt, should be fine.
I agree with you about your water paramaters, they look good.