Quote Originally Posted by CarlD View Post
You only need to worry if the calcium level is too high--then your water can get cloudy (but it's still safe).

Pool stores will give you an AMAZING song-and-dance about why you need calcium in your vinyl pool. It's nonsense.
If you have a heater with a copper heat exchange and your hardness is low it can destroy your heater. If there are any exposed metal parts in your pump or filter then your water when it is too soft (below 150ppm) your water can be corrosive as well. Most newer pump and filters won't have any exposed metal though and you should be find. If you have a vinyl liner and no heater 150ppm is a good point to stay at if you have a heater I usually recommend 250ppm just to be safe.