RO/DI water bad for tank?

[hockey nut]

Well, I'm still not sure what exactly caused the problems but my nitrates in two tanks are high and thats after 2 water changes. either something died in both tanks and I missed it or something got into the water. I'm testing the RO water as we speak . I have a not so funny feeling that something got into the water.
Regarding the Ro. I let it run for a couple of hours before I used any water on day 1. I mix my salt and water over a 24hr period and match temp and salinity as best as possable before I do a water change.
It also has now occurred to me that when I was away for a week I had my neighbor feed my fish and as much as I was thankfull for her doing that there may have been a small chance she had creams etc on her hands. HHHMMMM. I'll get to bottom of this yet. I'm sure the Ro is fine but I'm still going to test.
Thanks for all your help.