TDS's ...what do they do?

[chicki]

Can anyone tell me really what harm water with high TDS does to the reef tank? I have a RO/DI unit that I've been using faithfully for my reef tank. I set up another tank and put softies in it with fish. I decided to use my well water for it and it's now 5 months old and things look great. My well water has a TDS reading of about 650??
Thanks