i just use the water that comes out of the tap. for make up water and salt mix. but some areas have really bad water so its not always a good idea. if you get a read on a test thats not good you would have to find the problem and fix it. in fresh water you could add cems but you should not do that in salt water reefs.
ALSO read lots and do lots of research. .
if the salinaty is low you could add more salt water if its hi add more fresh water. if the ammoina is high something has died and you need to find it and get it out.