My TDS meter crapped out a while back and I never bothered to replace it. Recently, after changing all my filters, I decided to check it again and it was 230!! (my tapwater is 250). My RO/DI had been performing normally, by all appearances. it turns out there was some major problem with my membrane.

In all likelihood, I have basically putting in 5 gallons of tapwater into my tank every week, plus more from my water changes. It is amazing I don't have more problems than I have.

I just want to remind everyone that the only way you can check your RO water is with a TDS meter. Just because you change your filters often doesn't mean your water is good. TDS meters are cheap:

http://www.marinedepot.com/aquarium_...ts.asp?CartId=

And if you get the "inline" one, you don't even have to do anything other than to turn it on every now and then. I've checked my inline one against a regular one, and they both read the same.