TDS's ...what do they do?
 

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

R/O Units remove UP TO 90% of the total dissolved solids in the water, the DI unit if you have one will remove about 99.99% of what gets past the R/O unit. The R/O unit saves the DI filter so you don't have to spend $$$ replacing the DI medium all the time. The harm is not knowing what those dissolved solids are. Heavy metals? sodium, carbonates etc....

The other thing a TDS Meter is good for is testing the water that comes out of your R/O unit, whent he TDS start to rise you know it is time to change the membrane.